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.
Is this on the packaging? I have not been able to find any of this information online.