Are you using Photoshop 32 or 64 bit version?
Is you display driver up-to-date?
Try a smaller size photo format such as jpeg.
You may also try this:
Edit > preferences > performance > GPU settings > Advance > at the selection box select "Advanced"
If your work is large 4gb ram may not be enough, 8gb is better and inexpensive. When working on Photoshop, always close all other applications and clean your cache.
Let us know