After installing Yosemite, my iMac runs so slow.

Thanks Eric . . . I'll try that. BTW, I believe I can go up to 8GB. Do each of the slots need to be the same size?

Rod, I too own an early 2009 24" iMac. Mine has 8GB RAM and DuoCore 3.06GHz CPU and 1TB internal HD with plenty of free space. Here is the Apple info page for RAM upgrades on that model, which directly answers your question about size:

iMac: How to remove or install memory - Apple Support

But I'm afraid to be the bearer of bad news on the speed front. Despite my having 8GB (the absolute maximum RAM for this early 2009 iMac), the machine is so slow at times with Yosemite (every version through 10.10.4) that it is almost unusable. I use this Mac at the office. I also have another iMac at home which is a late 2009 2.8GHz QuadCore with 1TB HD and 16GB of 1333MHz RAM. That machine hasn't slowed down at all from what I can see, after my upgrade to Yosemite. Then again, my iMac at home has a better CPU, better GPU and double the RAM. And no, for those wondering, the internal HD on my March 2009 iMac is NOT failing. I've checked the Console logs and EtreCheck. Other than it being painfully slow under Yosemite, there is nothing else wrong with this early 2009 Mac. And by the way, this same early 2009 iMac ran fine and was very usable under Mavericks. The only thing I can see is that Yosemite is just a dog on certain older Macs like this. It's a real shame too. One can only hope that El Cap will make it usable again, but I'm not getting my expectations up.

Anyway, I hope this helps.

JD . . . I added 4 GB to increase my RAM to 8. It's hard to be objective about speed. I never measured or timed anything. It seems to run a bit faster but maybe that's because of the expense and effort and I want it to. It seems to boot faster and some applications open faster, but otherwise, I'm not really sure Yosemite is good for this machine. We'll see about El Cap . . . I'll wait a while before upgrading. Thanks for your help.