i do not have one on my RV-4 and would not put one on a RV-4. first, with the standard fuel system layout, the drain on the tank is the lowest point in the fuel system, and that should be where any water will be. second, on the firewall of a RV-4 is a very hot area and it will make for a great heat sink and possible source for vapor lock.
I put a one of these in the fuel line after the fuel selector, before it went through the firewall.
since my plane lives in a hangar, and never gets parked outside, i have never found any water in my fuel system.
I get water in the right tank of my warrior, when i find it, its from the fuel tank drains, I have never in 20 years of owning it found a drop from the gascolator.