Have you measured the width of the 4 strip before sub-cutting? Does it measure 10 1/2". Perhaps your seams are a tad more than 1/4".
Or - have you tried a scant 1/4" seam. Maybe the 'fold' of the fabric with so many seams takes up that bit extra when all is put together. I sometimes find using a finer thread in the bobbin makes a difference .