If you're getting months from a 4040 membrane (you say filters but I assume you mean membrane) then there's a problem that a second 4040 won't fix.
I struggle to remember but I think my current 4040 is roughly six years old. Input is 300-400 depending on season, output 9-12 depending on temperature, season and whatever else makes it change. Its lowest was (I think) 8 when brand spanking new. I think I went through four or so in my first couple of years. I lose track now, as I'm an old fart, but that all sounds about right.
I'm boring on the subject but I think you need to bleach your system to eliminate biological contamination. It's a PITA but it's much better than spending £250 every six months. You also want a dedicated tap with a hose permanently connected if your setup allows it, so contamination doesn't get into the system. If I disconnect the tap for any length of time, I'm very careful about where the hose connector sits.
Vin