4" are 101.6mm, though overall they're more like 120mm. The situation for me is quite similar (initially placed on/behind the PC desk) However my build is rather wide (20cm) due to the double MDF of the walls. Not sure they're necessary, though (would shave off 38mm).

The issue are usually the woofers, since these need displacement. 4" is already quite small. Maybe look for designs with passive radiators on the front and the woofers on the side? (I'm not deep in enough to make concrete recommendations).