Why four is harder than three or five
An even count has a gap where the focal point would be. A row of four solves that by making the rhythm itself the subject, and a 2 x 2 grid solves it by making the block symmetrical on both axes. What rarely works is four mismatched sizes scattered without a shared edge.
Row arithmetic
Four 11 x 14 frames with 2.5 in gaps measure 4 x 11 + 3 x 2.5 = 51.5 in across. On a 108 in hallway wall the first left edge lands at (108 - 51.5) / 2 = 28.25 in, and each following left edge advances by 13.5 in. Those four numbers are the whole install.
Grid arithmetic
A 2 x 2 grid of 12 x 16 frames with a 2.5 in gap is 26.5 in wide and 34.5 in tall. Grids are unforgiving: a 3 mm error in one hook shows against the neighbouring edge, so mark all four positions from a single datum rather than measuring each frame from the one before it.