Seating distance is a personal thing (not everyone sits in the same place at the cinema), but Sony recommends you sit at 1.5 x the screen height back for 4K, which for a 106" diagonal 16:9 screen would be 6.5 feet back. For cinemascope movies (on a 16:9 screen) at that width that's actually fine, but at that distance screen height can be an issue for taller 16:9, especially if it's too high, and is more like being in an IMAX theatre. That's fine for IMAX but not so good for standard 16:9 stuff.
So what you could do is get the projector, project onto the wall and experiment with screen height and seating distance to see what works best for you, and then put the screen at the height you liked with the seats located where they felt most comfortable for prolonged viewing. For 16:9 stuff you may find that 2x the screen height is better, so try something like 8.7 feet back as a start and then move the seats closer or further away to see what suits. THX actually recommend 2.4 x the screen height which would be 10.4 feet back.
As you're not going to be watching 4K all the time you may want to see what HD looks like as well.
Although with 4K we can sit closer, it's possible that vertical viewing angles can start to become more of a problem than image quality issues, so having the screen at the right height (not too high) may be something to consider. A bit like having a flat panel above the fireplace and not being able to recline to see it comfortably.