I'm wondering if you mis-wrote the dimentions. You have a 4 block x 6 block top. Are the blocks about 15" or 12 "??
If you have any fabric left, I would make another block, detach one row from the long side, unstitch the blocks and re stitch in the proper orientation, add the extra block and then add it to the narrow side. Now you would have a top that is 5 blocks x 5 blocks--much easier to work with. Now the top would be approx 77x77 or 62x62--depending on what size the origional blocks are.