Scum, start your run at a corner, and stop before you get to the middle. Start at the other corner, and when you get within a foot of where you stopped the first run, start pleating the edge. Just fold the edge to take up an inch and a half or so, and stitch right over it. Check to see if the edge lengths are the same. If not, add another pleat a couple of inches further down. Eventually, you will take up the extra 4" of length.
If I had a differential of 4", I would just add another dart. Either way, you have to adjust with pleats to align the edges. Just make sure your adjustments are between the corners. I do this on all four edges. If you always start at the corners, the corners will always align.