I have these stats, but is the system really using almost 4GB?
This Inactive memory... Can't I use it? How should I interpret these stats?
I suspect you probably are using Mac OS 10.7 which is very memory hungry. As noted by kccqzy, the purge command may reclaim some inactive memory if you have XCode installed.
Something you should try is closing and reopening applications which are memory hungry or have a tendency to leak memory (Safari, Firefox). Alternatively, restarting your computer would also accomplish the same though take considerably longer.
To answer the question, are you really using 4GB of memory. Yes. Technically more if you take note of the "Swap used:" stat.
Yes, you can. Fire up Terminal.app and run
purge. It will force disk cache to be purged. It is likely that the inactive memory will become much less and more free memory will become available. A temporary side-effect is that your system will be a bit slower just after running
EDIT: If you cannot find the command, you may have to install Xcode, which is available for free from Mac App Store or Apple Dev Website.
You're only using three GB, not four. Both Inactive and Free memory are available for applications to use. The difference is that Inactive memory is memory that used to be in use. It will retain its contents until it is needed for something else. That way, if the same program that used it before runs again, OS X doesn't need to reload anything that is still in memory. Free memory, on the other hand, is memory that currently has nothing of value in it.