#baatarthefirst

Now I didn’t go into detail, but this probably takes off a whole point for me, because I don’t feel like we should have to read extra material to understand what’s happening in the main plot line. Is this the first time we’ve ever had to read extra material not for bonus information but an understanding of the main story?

Hello, yes, I love answering questions in good faith. Can't promise that I'll have a satisfactory answer, but hopefully I can provide a not overly long one, for once!

Through the Moon (the graphic novel of which you speak) is the only supplementary material (thus far) that's been released that isn't a prequel, so in that vein, I'd say yes. The graphic novel does explain that Rayla left (and shows her leaving) before the 'reveal' drop at the end of 4x01. She left approximately a month and a half after S3, so right around / on Callum's birthday (which, we didn't know it was his birthday in the graphic novel itself, so that's almost undoubtedly something that got added to S4 for Callum&Viren foils and general heartbreak reasons).

However, to me, and this also due to having friends who watch the show casually, or have not read the graphic novel Through The Moon (TTM)... I genuinely do not think you need to read it unless you don't want Rayla leaving to have been a 'surprise' (which some people like to know spoilers ahead of time, which is fair). Given that there's a timeskip and an arc wrap up (and that we actually got to see them be a cute couple in S3, not just at the last minute) some sort of shift was probably to be expected. All the plot context TTM is given pretty early on in S4, but there are also fun little connections to make as you go on rewatches, for example:

4x01 reveals that Rayla is not only not at the castle, but MIA, and that the boys don't know if she's alive/okay. The context from TTM is that her risk taking decisions were already a concern for him, which can probably be deduced from the show's canon anyway, given that he jumped off a mountain for her.

4x01 and 4x02 also show that rather than being mostly curious about unknown things like before, they actively make him agitated/anxious (Soren's secret keeping, the blank book, trying to decipher the mirror). TTM adds context to this in that Rayla told him she would let him come with her, and then went back on that promise, but canon already makes this clear in 4x01 with "I don't even know if she's alive"; he has no answers about Rayla. That is also undoubtedly one of the reasons he's agitated. However, while her lie is insult to injury, the show never brings it up as a reason he's mad at her; it was always her extended absence: "So you disappeared for two years for no reason at all?" / "Well, maybe you might know that if you'd been here the last two years." Again: the lie isn't an actual thorn in his side (seemingly) so it's a thing from TTM that is omitted, because it isn't emotionally relevant.

Then she shows up and we learn, specifically, that she went to hunt down Viren. Rayla has always been a paranoid ("Sooner or later, we're going to run into someone from 1x06, "There's danger coming, I know it" from 2x01) person with an independent streak. Rayla leaving to kill Viren because she doubts her own abilities, when she spends most of the first few seasons doubting them (particularly in S1 and S3) and fearing failure ("You let him live but killed us all") is also very in character. One of my favourite little details is that Callum struggles to say Viren's name at all ("It's something Lord -- the previous high mage left behind") until Rayla shows up ("So you must have found Viren, huh? Was he dead after all?") because that trauma - Rayla was so sure that she failed that she couldn't trust they were safe - is so intertwined.

A lot of S2, as well, spent hammering in the fact she didn't want to lose Callum (to Soren and Claudia, to his pursuit of magic, to dark magic, to death). Her leaving him behind to protect him is not something that's a big logic leap to make, I think. After all: "It's okay, Callum. I'm not asking you to come with me. Protecting that dragon doesn't just feel like the right thing to do, it feels like the right thing for me to do" (2x07) and "But... I'm not going with you. You and Ezran should take Zym. But I can't leave" (3x08) and her almost confession last but not least: "Callum please, come back! You've got to stay with me [...] I can't lose you like this, you mean too much to me" (2x09).

TTM adds context - the nightmares about Viren ruining the life she still had, and her and Callum's increasing mutual fear of losing each other. They quite honestly don't even break up. Now, I don't think things would've gone well if Callum had gone with her (relationship wise) given their differing priorities in TTM and also how they show love (for Rayla it's protection, for Callum it's togetherness), but the last conversation they actually have in TTM is

Now, I'm not exactly expecting a casual reviewer to remember and notice all the tiny things, but I am expecting them to remember major plot points, specifically that Rayla really struggles with letting people help her (1x06 and 3x04 both leading to disasters with Zym being in danger because of it) because she doesn't feel like she deserves it, and that she has a tendency to be overly self sacrificial (too many episodes to count).

Her trying to save the dragon in S2 was heroic, yes, but also stood out to me specifically as "This tendency of risking herself is going to be something that she and Callum have to confront and work through, because it is diametrically opposed to them functioning as a long term couple/team". When she wants to stay behind and die for her parents mistakes in S3, the only reason she doesn't stick with that is because she learns that they didn't run, not that she didn't have to die even if they had. So something like this, even if not in so dramatic a way was something that I'd been expecting for a few years now. S4 and Callum's anger at her abandonment is the first time we've seen this tendency be solely in a negative light, in a lot of ways, which is really exciting, and I don't know how much they'll get back together until Rayla has had the epiphanies she needs to.

The biggest thing to me that TTM adds to S4 is that Callum isn't just upset at Rayla for leaving him, but that he's mad at her for leaving without him, specifically. He wanted to go with her, he didn't totally believe in the mission but he wanted to keep her safe, citing "I let you go into the Moon Nexus alone" (earlier in the graphic novel, and not without protest) "and I knew right away I made the biggest mistake of my life. I could have lost you." But again, I expect this sentiment to be something that is far more readily explored in S5, and is also something you can kinda piece together given how often he runs after her in the first three seasons, and him reading what's implied in show and confirmed through TTM to be her goodbye letter because she didn't think she was coming back (4x01 credits):

TDP official also in the lead up to S4 released the goodbye letter, which we'd never read before, and it broke all of our hearts.

Submit a sex dream your character has had about mine.

Small outbreaks here or there usually in the rural areas of the recovering Earth nation or Earth Kingdom or whatever it was that they were calling themselves now. Kai didn't think even the know it all elders in Republic city knew what they were doing about the mess Kuvira created. So it came as no surprise that such outbreaks were looked at as simply a strange new symptom of food poisoning, or the flu, or even a disease carried over from another continent. At first people blamed the Fire nation, their shifty pass still harbored hatred in the older folks, and naturally it ran down their bloodlines like icy poison.

 Kai thought it was something to be concerned with, and so when larger cities began popping up with these 'outbreaks' he made it a priority to investigate the happenings before they reached his new home of Republic city, and more importantly Air temple island. This journey led him to Zaofu, a well armored location that while surrounded by outbreaks and to put it frankly the walking dead, had somehow outlasted the biting curb of this 'special illness.'

 The young airbender glided now high above the skies of Zaofu, having come from Republic city. On his journey he would see scattered living dead milling about, striving to survive only on primal instinct alone. However as he neared the giant metal city it became apparent the numbers of once prospering Earth Kingdom people were now clawing over one another to grant access to the innards of Zaofu's city. If it were any other city the hoards would have already overrun the compound. Thankfully the metal walls were high, and tough...and the walking dead had no ability over their bending, if they harbored any in their past life.

 He was aware of being watched like a hawk by the guards on the watch towers and made it a point to circle the city a couple times to announce his presence. Once he was reassured it was alright to land, the young air bender slowly descended and dropped to what could only be assumed as the town square. He took notice of a familiar face approaching and bowed briefly his hands folded politely in front of him the picture of the perfect air bending greeting.