I would say that it is a bit like DOS and Windows. The Beo4 is faster to use, more reliable and never malfunctions. The Beo5 is very user friendly, joy-full to use and can be customized beyond belief.
The Beo5 is much more fun than the Beo4. The Beo4 is a commander, where you hit the buttons need for control, like LIGHT, LIST, 4 and GO - to turn on lamp no. 4.
Instead of "commanding" on a Beo4, I choose a lamp on the Beo5, and then get the options that applies to the lamp. Look at the pictures below (Mere = More, Mindre = Less, Tรฆnd = On, Sluk = Off).
Another example is in the kitchen. When I choose radio on the Beo5 I get the logos of my 8 favorite stations on the screen. Of cause you can lean all the program numbers, and commanding on the Beo4 is faster than choosing of a Beo5, but is just not the same experience.
I would buy a Beo5 again any day, but if you do not have link rooms, lights etc. but "only" a simple one room setup, it might be a little overkill.