Open your image in photoshop, then go to the "view" menu and select "Rulers" (unless it's already checked).
Then click and hold on the left ruler on the left side, and drag to the center of the image... this will create a guide line. (if you have "View > Snap To > Document Bounds" selected, the guide line will easily snap to the center when you get close to it)
Then do the same from the top ruler to drag a second guide line to the center.
Now use the select tool (what photoshop calls the "Rectangular Marquee Tool") and click/drag from one corner of the image to the center... it should snap to the guides you just created.
Then simply copy and paste the part you selected into a new image... and do the same for the other three parts of the image.