Your 4 pictures will be on separate layers. Make these 4 layers active (click on each layer while holding the CTRL key on keyboard). Now, on the move tool's option bar, at top, click on align to vertical or horizontal centers or the edges.
That's so useful, thank you so much. I'm taking part in an end-of-year class exhibition and have been struggling a bit with Elements and you're help has really solved a lot of headaches. But I may be back for more!