Waterdeep: Dragon Heist seasons missing chapters?

I just purchased Waterdeep and was looking through the modules and noticed that all of the addon seasons are missing different chapters (pic of game with summer attached). I'm not sure if I'm missed adding something or of this is intentional.

If I remember correctly, they each put their own spin on chapter 4, then each gets their own chapter. It should explain it in the material for chapter 4 in the base module, or one of the other handouts. 

I noticed that but I thought that there were 8 chapters and in each of the season addons there is only 1 chapter added excluding the 'new' chapter 4 in each. So 1-4 base module plus one chapter for each season ie. summer only has chapters 4 and 6 and seems to be missing chapters 5,7, and 8.

The first part Chapter 4 is common, the rest of it is added by virtue of season chosen. Ch.5 Spring Madness Ch. 6 Hell of a Summer Ch. 7 Maestro's Fall Ch. 8 Winter Wizardry Each appropriate addon will install one of those chapters in any given game. The print book has a Chapter 9, which is actually the Handout called "Volo's Waterdeep Enchiridion"

So there are supposed to be five chapters total with a repeat of chapter 4 (for each season)? Thanks I was looking at them and thinking that I was missing something due to the chapter numbering. I didn't even notice chapter 9, I'll add it to the game and look into it, is there anything critical to the game in there or is it more of an addition of information to the story? And thank you both for the helping me figuring this out!

Chapter 9 is installed with the base game. It's basically Volo's guide to Waterdeep. WotC published it at the end of the book, but it would be confusing for the base game to add Ch. 1-4, 9. So Chapter 9 is there, but unnumbered, and presented earlier, with the rest of the introductory material. In print, Ch. 4 is all seasons jumbled together in one chapter. Roll20 has split some material out by season, to make it easier to load and run. If you create a game and add one season, you have everything needed to run that season. If you want the whole book in one campaign for reference (not performance-recommended for play), create a base game and add all four seasons. You can then read everything. This technique is described in the introductory handout at the top of the Journal Tab.

I agree, it's a little confusing, but the decision was made to present it this way due to the unusual nature of the module. WotC wrote it so that GM's could decide which plot to run. In a VTT, the unused material just adds campaign load. But it's all available one way or another.

Plus the chapters have some of the same elements with different effects/descriptions/statistics, and will overwrite if you load all the chapters into one game.