Many blocks have a center that would be big enough for signatures, you could put the 4 with signatures in the middle, or placed in the corners, however they would look best. Or you could use a siggy block and most of them look pretty and you could make the whole quilt out of them.
http://www.quilterscache.com/S/SiggyStardustBlock.html
http://www.quilterscache.com/S/SignatureBlock.html