I'm enjoying phase 3 and could probably stick with this forever. I was a HUGE carb eater before IP, but now I don't really miss them at all. Is it really necessary for me to go to phase 4?

Really?!
The only problem with staying on Phase 3 is that it is still a bit calorie restricted...although you have a very low weight, so it may not be for you. To be sure, I would do a general calorie estimation for your typical day and plug it into your lean body mass calculation to see if you are meeting your daily cals to maintain weight. If so, your golden If you are under your daily cals, you may want to add a fruit or two (or perhaps a dairy carb).

I'm going for my WI tomorrow so I'm gonna ask and see what they say. I'd be ok adding a fruit or two and having a cheat day every once in a while.

That's great to hear that someone is loving it that much in phase III. Hope I'm feeling the same. Let us know what you find out.