I've never been to this place and I was wondering how difficult it was compared to the "real" game (at the same level of difficulty). For instance I killed Diablo in Hell without much trouble, would Whimsyshire in Hell be a bigger challenge?
I have only experienced Whimsyshire on Normal difficulty, but the monsters there were on par with what you face in Act IV. I brought a level 24 friend along and he was getting stomped pretty hard (but not oneshotted). I, as a level 32ish wizard at the time, found the creatures appropriately challenging to my level.
I would assume that in higher difficulties, Whimsyshire is similarly scaled to be roughly equivalent to Act IV of that particular difficulty.
If you can do the difficulty higher on Act I with minimal problems, I'd say you are ready for that difficulty of Whimsyshire.
So if you can take Nightmare Act I, you are ready for normal Whimsyshire.
Note that with a group of 4, this can change drastically. Soloing will not be a problem at this point however.