About the nap...well if she still falls asleep easily during the day then yeah, I agree she totally needs a nap...but maybe she's falling asleep because she's waking up at 4am?? But actually I really do think that this 4am thing is more about a routine of getting you for some snuggly quiet time than about sleep, however.
My opinion is that you need to be a little tough and just start letting her CIO. I know that at 3 it's way more than just crying, but honestly, I think she needs a little tough love, especially if you have a baby that will be waking up multiple times as well. I would just pick a day and tell her that you will no longer be coming in there in the mornings. Tell her that if her doll falls she can pick it up, teach her how to pull the covers up, etc. Maybe even do some practices before or after nap time.
I know that it is really hard to listen to her whine, call for you, cry, etc. but you are going to have to smother yourself with a pillow and let it happen. It is really unlikely she is going to wake up her baby brother. At 2 weeks, those babies sleep through everything! Also, eventually (and it won't take that long) she will get used to the sounds of you getting up with him in the middle of the night and that won't wake her up (not sure if that's happening or not), it just takes time.
I think that once she realizes you aren't coming in any more, she will sleep longer and be happier when she gets up.