Heyy,
I've done five AS levels (Plus GS) this year, and it's not been any more difficult for me than many people are finding four. I did Photography, which was exceedingly time consuming, German, Eng Lit, History, IT and General Studies.
The only thing that's really an issue is that you have so few frees, with mandatory GS and tutor at my college, I was down to about 3 frees each week plus a lunch break everyday. So with subjects like sciences, where there is a lot of work...this might become quite a problem. To be fair, I took three essay subjects and a coursework subject which isn't light, and didn't struggle especially.
Honestly, Geography doesn't sound like it is that compatible with the sciences, perhaps focus on getting 4 As at AS, rather than 5 Bs? It really depends on if you think you can cope, and if it's worth your time. Everyone I know who does Geography seems to hate it :/
Sorry for the ramble... hope this helps