Why is my memory on my mac almost all used?

Open activity monitor, make sure it looks like this, and post a screenshot.

 

I would get rid of that app that you have running. Go look at Activity Monitor and find the process being run and see which one is using all the memory.

HTH

 

To OP: Does it make any difference though? Because of this, do you experience any major slowdowns? If not, I say stick with what you have. No one here can help you with freeing up your RAM. If you are that interested in freeing up your RAM, go to Mac App Store, and search for iCleanMemory, Memory Sweeper Pro, etc. It will help you for short period of time. If you want more free space, go and buy the 8GB RAM. Worth the upgrade if you do RAM&CPU intensive work.