Taking your points one at a time...
"1) a system that keeps me from having to go through "the waste tank is full" stress, and the associated fun of hunting down a working pumpout"The Lectra/San would certainly do that...it doesn't store ANY waste, but discharges it overboard a flush at a time."2) a system that is odor-free as possible. The Lectra/San is 100% odor-free. In fact, ANY odor is a signal that it's not working. "3) a system that is simple, fewest parts to break or replace as possible.The Lectra/San is as simple as it gets--head to unit, unit to thru-hull (and maybe a couple of vented loops, but no big deal compared to maintaining a holding tank)--and requires almost -0- maintenance...only an annual cleaning (which doesn't require taking anything apart...see the directions in the manual online)."With my installation on a catamaran, a Lectrasan may pose some problems, but I'd imagine they could be worked through."If there two heads, each in separate hulls, one L/S can't service both toilets. However, neither can a single composting toilet serve both heads...so that's a wash. "my question is if anyone knows if a crew of 4 would overwhelm the Airhead?"It depends on what you mean by "overwhelm." Can it handle the volume of solids? Yes. But it's the liquids that are the problem. The average adult uses the head 5x/day....average liquid output, including the liquid content of solids: 8 oz. That means each person aboard will put 40 oz of liquid a day into the toilet...that's 160 oz (a gallon is 128 oz). Those liquids have to be drained off--either into the one gallon containers offered by Airhead--which you have to store somewhere--or into a holding tank (so much for avoiding plumbing and pumpouts) for composting to occur. You do the math: people x days aboard. And then you decide which makes more sense for your boat, in you waters.If you were on the Great Lakes or any other inland "no discharge" body of water, I'd say that the Airhead is a system worth considering...but in coastal waters, why have to deal with waste aboard in any form if you don't have to?