marsha, For 4.5" height UF, if your strings are 1.5" or less in width, you need about 5 3/4" long strings. If they are wider, you'd need them even longer. If you shorten the block to 4" UF, then I think you'll be OK.
I pieced my "too short" strings so I could use up as much as possible. I've probably got about 1/3 of the strings pieced. so like 300 out of 1,000 for each set. Most of my strings were less than 1.5" wide, so I needed a lot more of them.