4 GBs of RAM is not nearly enough to run both OS X Mountain Lion and PSCS6.
OS X, by itself, can use between 3 and 4 GB of RAM for the OS to run smoothly.
Leaving only 1 GB or less for running Photoshop.
Photoshop is notoriously CPU! GPU, RAM and HD intensive.
Your iMac can take a max. RAM amount of 6 GBs.
Install the additonal 2 GBs of RAM. Hopefully this will be enough to run PS sufficiently.
You can purchase reliable RAM online from either Crucial memory or from OWC.
Here are the specs for your iMac's memory
Memory Slots 2 - 200-pin PC2-6400 (800MHz) DDR2 SO-DIMM
Here is the procedures to install the additonal RAM memory, yourself.
http://support.apple.com/kb/HT1423
Good Luck!
;)