iMac running so slow, please help!!!

Okay, so here is my issue. After rebooting my iMac everything is fine, no problems with opening apps, clicking any buttons... Nothing. I would say then roughly about after an hour and a half of use (mind you I make sure to quit all applications I am not using due to frustration with the snail pace) I usually walk away from my computer and it puts itself into sleep mode. Then I go to turn the computer back on, no problems with reboot... But THEN IT HAPPENS... I will go to click any button in the task bar, and it will take roughly 4 seconds before anything will pop up... Open any applications? Forget about it your waiting easily 10 seconds for anything small to open up.. Forget about any big apps like Illustrator. I have tried to every type of cleaning imaginable from repairing permissions on down. RAM isn't really the problem either as I still maintain about 2.7 GB of RAM (I am currently running 4 as of now). CPU percentage seems to be okay, it stays roughly about 80% idle.... But it takes forever for anything to happen.. I also thought it may be hardware so I ran every test I could think of from memtest on down. Everything came back O.K.. Has anyone had any problem like this before as I would really like some help...

 

I have a close to a tb of free space left on my HDD... My mac is a 2009 21.5 3.06 GHz Intel Core 2 Duo with a tb of memory on the HDD.... Even when my computer is running slowly I have roughly about 2.4 gb of RAM avail... I will try zapping my PRAM and see if that does anything. There is no spinning beach balls or anything, just like a 3 second lag when clicking anything. I agree 4 gb is not a lot, but my Toshiba netbook with 1 gb of RAM didn't even run that slow even while running almost every application imaginable.