First we need to know if the 60.5 X 48.5 is the FINISHED size of the quilt or the raw size.
I'll take a guess and say it's the raw, so the quilt will finish at 60 X45.
60 divided by 4.5 = 13.333, so you want 14 the long way which will require the quilt to be 63", and 45 divided by 4.5 is 10 the short way, which will fit perfectly. You need a finished inner border added only to the 2 short ends to bring the long side up to 63", so 1.5" additional on both sides. You would cut the strips 2" to finish at 1.5.
Looking at your quilt dimensions, if you could find a border that finishes at 2, 4, or 6 inches, you wouldn't need to add an inner border at all because the math would come out perfectly. (2, 4, and 6 all evenly divide 60 and 48).
remember to make 4 additional blocks for the corners.