4" is about te average screen size these days. Just about all PocketPCs have some form of note taking application, the current ones all have NOTES and also Pocket Word.
If you are looking for hand writing recognition then most PocketPCs offer this with their Transcriber feature but not everyone likes using that. There are a lot of 3rd party software vendors out there who make their own recognition software. A popular one seems to be PhatPad from PhatWare over @
http://www.phatware.com/phatpad/ .