You can get 4T/5T overnight pull-ups online, but if you need to have a brick-and-mortar solution, size S/M GoodNites will work fine.
We haven't had issues with heavy wetting (and my DD tends to be a heavy wetter) if we remind her the Pull-Up is for overnight only, not to catch pee while she's awake.
I find the 3T/4T overnight pull-ups to be too narrow across the bum; my daughter is also 4 and mostly wearing size 5 clothes. (I bought her a few things that are size 6 to get her through 'til Spring.)
==========================================
Liz
DD (3/2010)
"Make mistakes! Get messy!" - Miss Frizzle