Jorgen, the 5th port is in reference to the port on the bottom of the Apeks DST and its clones.
Bison, we've been asking for over a decade, it is frankly a lost cause, Poseidon has basically sold out to the recreational market at this point. It's unfortunate because the turret style first stage offers superior twin-set hose routing as well as sidemount, and makes CCR diving a lot easier as well.
Best scenario is to take a first stage of your choice, remove the environmental seal, add an OPV *assuming you already went away from the absolutely ridiculously priced Poseidon hoses*, and have a tech who is certified on both pieces of equipment to tune them properly. That's how I've been diving mine for the past 6 years in sidemount. Stick the Poseidon first stages *which are brilliant, just the wrong design for the way the industry is headed*, and use them for deco/stage bottles. That's what most of the guys that I know that are still diving them have done. They sat in boxes for years while we waited for them to open up the distribution channels for parts in the US, they came out of the boxes about 3 years ago, but most aren't in matching pairs anymore.