If I understand this, you did 4x4 of the 6" which should give you 23.25" if you used .25" seams, right? I think I would have squared up after each set of 2x2, but I'm sure it's too much work to take that apart. If you square up the ones you've got now, will they look right? If so, then yes, you can put them together and sew a sashing on top front and back. Another idea might be to try to make them wonky - if you have a lot of sashing material. You might need to add a few pieces if you do it this way. Hope this helps. Also, try sewing sashing on one direction and then the other.