The port on the left goes to the dome. The center port goes to the bottom of the gate. The right port goes to the compressor housing.

You don't really need to buy a 4 port. Just replumb your 3 port like the pic above. As far as going down to zero, not sure why you'd need that. Anyway, plumbed this way I cracked my 22 psi boost cut the first couple times until I got the DC figured out. If you decide to replumb, start low and work your way up. Be sure to drill a relief hole in the line going to the done (.030 or so) so pressure doesn't get trapped in the dome.
Boost curve is on the bottom panel in red.
​​​​​​​

 

For the record… if you setup the 3port properly you can get the exact same boost range as a 4 port with maybe a hair less resolution. You just need to plumb it properly with a tiny bleed hole on the top port.

 

Exactly what I was referring to. Thanks Force for throwing up the pic!