There's a fine line between having fewer theaters with more seats, more theaters with fewer seats, and also cycle times.
They probably only have so much land they can allocate to the facility (especially if they do want to allow for a phase 2), so the footprint of the building is fixed. By putting the 4 theaters into this building, that limited them on how wide each ride apparatus could be.
With 4 theaters, the guests will feel as if the line never stops, because they'll be able to alternate loading between the 4 theaters almost continuously, based on the show length. Think of Haunted Mansion - while it's an omnimover, the queue isn't actually loading onto the omnimover. This would be similar to Stitch and HM - except with twice as many show rooms.