Well, I dont know what to say here. I am in the same situation. Medium use kills my battery in 4 hours too. I even had my handset replaced thinking I had a faulty model. Then replaced the battery. But no change.

I just accepted this is what the N900 is like. I carry a spare battery when i know i wont be able to charge this beast. Very disappointing but what can you do.

P.S.
I also dont have any widgets on my desktop.
My 3G is usually connected but idle.
Bluetooth off.
Screen Brightness very low.

Interesting thing is that on my travels in march, i was in a country where only 2G was available - and my battery lasted me 10-12 hours with medium use. I know 3G sucks battery life but only 4 hours with 3G? something isnt right there. The N900 was built to be used with the latest technology. Being a premium device, most N900 users are in areas where 3G is available.