To answer your question: yes, Logic and Mac OS 10.6 can use more than 4 gig combined.
Some details: Logic, in 32 bit mode, starts to become unusable and will corrupt your projects at about 2.6 gig or so of Logic own ram usage.
Another thing: even in 32 bit mode, some things, EXS for example, that have their own separate and additional ram they can use as long as they are set up for it (for example, if you have 6 gig or ram, and you want to run 3 gig of EXS samples, plus a bigger session that needs 2 gig to run, you are ok).
Most current 3d party sample players have their own 64 bit ram servers available that don't have limits other than the entire amount if ram you have. If you had 15 gig or ram like I do, You could run Logic with 2 gig ram, the EXS server can be running 4, the Play server can be running 4, and Kontakt or VSL each running 3. Once the system gets close to using the entire system ram like this, you will find your session will become very sluggish.