My first question I would ask myself is - have I seen any other's doing it, especially retailers of complete assemblies? Answer; no. It would be easier to run water in and out of both filter housings and then straight into the inlet of the 4040 housing if they were fitted horizontally, so why haven't I seen anyone else do it?
I don't know the answer to this, but a couple of thoughts spring to mind.
1. All the housing brackets I have seen are designed to be "lift off" brackets so the filters can be removed off the wall/base board. So they rely on weight and gravity to keep the housings in place. I have always found it easier to take my r/o filters off the garage wall, take them to an outside table and unscrew the filter bowls to replace filters. I have also found it easier to put both a new sediment filter and carbon filter in vertically upside down. I can allow gravity to keep the filter in place at the seal and as I screw the bowl on, ensure the bottom of the filter lines up in the housing properly.
2. I use GAC filters on my system. The carbon filters are designed to work vertically. The water passage from the filter's inlet is down the outside of the carbon filter container, up through the bottom, toward the top and then out. This carbon container is of small diamt but tall. Inside this inner container are lose granules of carbon. In time I could imagine that this carbon could break up into smaller pieces with the turbulence of the water passing through it. If this does happen and the carbon takes up a smaller space, all the water passing through it will still have to pass through the carbon as the carbon will settle to the bottom. If it was on its side, then a gap could be left at the top of the container allowing untreated water to pass through.
IMHO do as everyone else does. If you don't and assuming I'm correct, then replacing a prematurely failed r/o membrane because of chlorine contamination is an expensive lesson.
It won't be an issue with the sediment filter to have it horizontal, but pointless.