how can I know the real use of my programs in memory. I use activity monitor but if I add up all the individual contributions I never get 4GB RAM
It is a good thing that you never get 4GB RAM. It would be very bad for your system if all your applications used all your real memory. There must be some free memory at all times for the system to run smoothly and virtual memory paging to work properly. I would be worried if your free memory is under 50 MB.
Apple has a support document explaining memory usage shown in Activity Monitor.
I like iStat Menus for giving me a quick snapshot of my memory/CPU/Network usage.
if there are some services that are wasting memory and which I do not need like google update, etc, and how can I deactivate them
You can see what processes are using a lot of memory with Activity Monitor and quit processes there.
Unless your Page outs value under the System Memory tab is very high compared to the Page ins value, or you get excessive disk thrashing, I wouldn't worry about it. I think the best way to limit memory usage, given the set of applications you are using, is simply to keep the number of tabs/windows you have open in Firefox to a minimum. 4 GB should be more than enough to run that set of applications in Mac OS X.
other advices for freeing memory
Frankly, in my opinion there's not much you can do besides limiting the number of applications/startup items you launch.
The most important thing is ensuring you have enough disk space for Virtual Memory paging. The X Lab has an excellent article explaining memory usage and how to determine if you have sufficient memory.