first, it's your quilt, you can lay it out any way you want to. Next, I like the single blocks with the light sashing around them but see the problem you encounter. I see a couple of options right away for sing the the single ones with sashing - if you want all your sashing blocks to be the same and balanced, make the sashing bigger all around the block to make it overall the same size as four of the smaller blocks, or...if you like some imbalance in your quilt for movement and interest, make the sashing bigger on two sides of the single block so that it's off center but still the same size as the four block square. Make sense?