The WM 4000 uses a salt-tablet based Secumar auto inflator mechanism, not the hydrostatic pressure-based inflator. Be aware that they have a very small fine print caveat on them that they are not designed for people shorter than 5' 8" or so. I believe this is because the harness is not height adjustable... so if you're short, like me, don't get it.
Thanks for the heads up, I am over 6' tall so not a concern there.
When everyone says built-in harness, is is one that wraps around your legs like a climbing harness or am I thinking incorrectly? Just trying to figure out what to do comfort-wise.