Sorry about the PC comment, just a little joke that passes for neverending fun with imaging users. The info below is directly about a PC setup, but my guess is that the Mac will be the same. I used Macs years ago, but it seemed that Photoshop was virtually the same on both platforms when I switched. Other programs were quite different, sometimes, but Photoshop should be the same.
Although you have 4GB of RAM, this should help. You do have extra drives, so this should be a default setting for you anyway. ** if you do change the drive letter for the scratch disks, then restart Photoshop to see the results.
The Scratch disk: When your system does not have enough RAM to perform an operation, Photoshop uses a proprietary virtual memory technology, also called scratch disks. A scratch disk is any drive or drive partition with free memory. By default, Photoshop uses the hard drive on which the operating system is installed as the primary scratch disk.
In the Plug‑ins & Scratch Disks preferences in Photoshop (under EDIT> Preferences>Plug-Ins & Scratch Disks), you can change the primary scratch disk and designate a second, third, or fourth scratch disk to be used when the primary disk is full. Your primary scratch disk should be your fastest hard disk; make sure it has plenty of defragmented space available.
*For best performance, scratch disks should be on a different drive than any large files you are editing.
*Scratch disks should be on a different drive than the one used for virtual memory.
*Scratch disks should be conventional (nonremovable) media.
You would also benefit from increasing the Memory Usage, especially since you have so much. (Edit> Preferences>Memory & Image Cache) It is listed as a % of your total RAM. Up it to at least 75%(opinion only) Test different settings and see if your Photoshop starts behaving properly.