You're almost certainly running a 32-bit version of 10.04. 32-bit OS's
can only address 4GB RAM (a lump of which you always lose to hardware
and OS ... and graphics if it's on-board shared memory, such as in a
laptop).

IIRC, W7 is all 64-bit now, hence being able to see the larger amount.

In order to benefit from the >4Gb RAM limit, you'll need to run a
64-bit version of the 10.04.