I'm also a solo consultant (really just a contractor), and I've been working 20 hour weeks for the last month or so. I'm finding that I actually don't feel productive enough, mainly because I'm working for a small unlaunched startup, and they are really itching to launch. I mean, I'm still feeling productive for those 4 hours, but when I'm in "the zone", I can focus for much longer. When I'm work on a personal startup or side project, I often spend all day on it, maybe with a few breaks for food.
I'm holding fast to my 4 hour days, but I'm starting to feel like it's not ideal to be the sole part-time developer on a project that needs to launch soon.
But I suppose this could probably be mitigated if they hired another 1 or 2 developers.