Nothing2NoOne, I've been a Mac user for just about 30 years, and I only used Windows when I was forced to during my employment with the U.S. Judiciary for fifteen years, through the summer of 2005.
From that point of view, I have to say that your computer sounds kind of puny, with insufficient resources.
Starting with RAM, 4 GB won't cut it these days. The minimum requirements published by Adobe are way too optimistic. Again these days 8 GB is consider the minimum. I personally run CS6 with 16 GB and others have 32 GB, 62 GB and even 128 GB install, I can only drool when I get to work in one of those machines.
20 GB for scratch is very, very little. I would even call it pathetic.
The rule of thumb I follow to figure out scratch space says to figure on 50 to 100 times the size of your largest file ever multiplied by the number of files you have open. I have seen the scratch file exceed 800 GB once, an admittedly rare occurrence, but it often exceeds 200 GB when stitching large panoramas and the like.
As an example—and stressing that I'm aware that others have even more scratch space than I do—I keep two dedicated, physically separate hard drives as my primary and secondary Photoshop scratch disks and a lot of GB free on my boot drive for the OS. I also have 16 GB of RAM installed.
Additionally, if you only have a single HD, i.e. your boot drive, you'd need it to be large enough to accommodate both the swap files of the OS as well as Photoshop's scratch.
You mention that you "had never had a problem with Photoshop until lately". Has the nature of your work changed lately? Are you now suddenly working on more substantial files than you were before?
What are your settings in Photoshop Preferences > Performance. Here are mine as an example, just to make sure we are referencing the same settings:
Sorry to tell you, in effect, that your computer sucks, but that's just how I see it.