Man this might be just me but I hate having drivers actually at ear level. An Ampeg fridge cabinet is only about 4 foot ball. A full Marshall 4x12" stack, the top cabinet would sit about 5'4" from the floor, and the cone of the top driver would probably be at about chest level for an adult male. What I'm getting at is that it's really unusual to have a speaker stack get all the way above your head. I've sat my combo amps on shelves that were chest level, bringing the drivers to about ear level and I hated the experience. It was sort of painful, honestly. I felt like I was afraid to dig in because the treble would make my ears explode. But I couldn't EQ the treble away either, because people do need to be able to hear what I'm doing.
To me, the ideal height for a speaker is such that the sound is hitting my instrument head on. That way, my instrument is achieving resonance with the drivers, and I get all that sustained goodness, but my ears don't have to get the full force of the speakers, but it's close enough to my ears that I hear myself very well. But when it's at my ear level, I feel like I start to play timid because the sound is hitting my ears as hard as it's hitting my strings, and I'm not taking full advantage. If you like 15s, and I do, I think 2x15" vertically is ideal. If you're gonna go 4x15", then go 2x2.