The ML board without trackball (4100?) wins the prize for 'smallest board that's half decent to type on', IMHO.
The 4400 is noticably larger (even ignoring the extra bit for the trackball).
I wouldn't like to use either full-time, but I use a 4400 to access email on computer B while my KVM is switched to computer A or C, and I used to use a 4100 on this 'living room pc' quite happily for years before I replaced it with a GHSS.
The trackball isn't great, but it works OK. That's about all that can be said about it.