It depends how big the documents you are working on are.I've been running Photoshop on a PC with 4 GB of RAM for years and never had any issues. Photoshop is not much of a RAM hog as say After Effects.
I know a lot of people take that stance, but depending on what the OP will be doing with the computer, 16GB may be a waste of money. Most people don't need that much. I guess I'm of the mindset that says it doesn't make sense to buy something that won't be of any use to you...even if it is cheap. Just my 2¢.Just get 16GB, its cheap.
You DID explicitly cite PS as the reason for your question!
4GB is okay, especially if you don't do multy-layer 300DPI 8.5x11" stuff (or Retina, which would be very close in approximation). I've 8GB in mine and can max it, especially if I have web browsers or VMs open... I just upgraded to 16GB so I can run VMs in the background...Currently got 4GB ram MBP. Going to upgrade the RAM as I want to start using photoshop (already get some page outs without using photoshop).
So... is 8GB enough, or should I go straight for it and just get 16GB?
Can you upgrade the 13.3" Regular (not retina) MAc Book Pro to a 16gbLarge print posters with complex layers, yeah, you'll want either the 16gb or not to be running anything at all else on your computer at the same time.
Web design you should be fine with 8gb, probably a little cramped on 4gb. Unless for some reason you need to keep a dozen files open at once.