Do you want to start out with 4 1/2 inch squares of fabric and make them into the 4 patch OR do you want the 4 patch to be 4 1/2 inch unfinished? If you want to start with 4 1/2 inch fabric squares just cut off 1/2 inch on 2 sides, top and side sides.... If you want the 4 patch to be 4 1/2 inch unfinished do what Charlee said.
Easiest way to figure what size to cut fabric is to start with your FINISHED size. If your 4 patch needs to be 8 inches when sewn into the quilt each patch needs to be 4 inches, now you need to add in the 1/4 inch for your seams--you need a total of 1/2 inch, (1/4 inch each side) so you would cut your fabric 4 1/2 inch square.
You can do this on all square blocks no matter the number of squares in that block.
Good luck!