The great thing about the XLE's is they work well at lower pressures. I start mine out at 200psi and they get up to about 300psi when really fouled. I have clamp style end caps and they've never leaked. I even got the pressure up to 450 once (actually stalled the procon pump) but nothing leaked anywhere. My guess is the size pump you are looking at would be just right for one 4" membrane. My pump is about twice the size for my 2 membranes.

One other thought, I've got a recirc loop on my machine which lets you achieve higher concentrate levels and helps with fouling. There is an adjustable valve that takes some of the concentrate and runs it back to the intake of the procon pump. I actually don't use it much though because I can make more permeate per hour by running the concentrate back to the storage tank and recirculating until I reach the level I want, then I divert it to my feed tank. It's more of a hands on approach since you have to keep checking the concentrate. If I want to "one pass it" I'll use the recirc loop/valve and set it and forget it.