I have the 4 and 5 "million dollar homes". The 4 is a tight squish for DSLR + something else. The 5 is big enough for DSLR and a few other accessories.
I originally bought the 4 for a point-and-shoot and compact video camera.
My sister bought an even larger one as a diaper bag, etc.
Have you been to Crumpler's home website? They have illustrations showing how much you can fit in each bag:
www.crumpler.com.au
Esther