Hi - Phase 2 replaces an IP meal with another animal protein meal so the body starts getting used to digesting that. Phase 3 wakes up the pancreas in a controlled fashion by having the kickstarter breakfast that allows certain amounts of the fruits/grains/dairy. It's when you fall out of ketosis. You can gain a little as the glycogen builds up again, but I did and then lost it, so just go with the flow, I say.
Phase 4 is what you maintain in for the rest of your life. You get to add complex carbs, and all the vegees you want. The key is not to have a meal with lots of carbs and lots of fats at the same time. Lunch has fats but minimal complex carbs; dinner has complex carbs but minimal fat. They are not as specific on how many tomatoes, for example, but I'm thinking that the knowledge and habits we picked up in Phase 1, such as watching how many times we have things like that, will carry over. Your body is supposed to be able to handle those kind of sugars by now, but I still want to be cautious.
They say that people tend to gain 5-10 pds over a year, so then they go back to 'boot camp' and do Phase 1 for a week or so. That's fine with me. If I can stay healthy and never get to my prior health, I'll be happy. Some people find that they just can't handle the complex carbs, or some of them, so they just stay away from them. Phase 4 is where you really get to know what your body can handle and what works for you long-term. It's scary but actually empowering too.
Good luck! You can do it!
The "Life after Phase 1" and "Maintainers" threads have lots of great info.