Well right now, ur only way to have DX9 in UE4 would be to write it in yourself, and this would be no easy task, pretty much the entire renderer would need to be re-written specifically for DX9 (no compute shaders what so ever) and simplified. Something Epic won’t do at this point for like 2% of users who will use it out there. Not to sound mean, its just the reality of the situation. Maybe someone will write a specific DX9 renderer for UE4, but it will most likely be a community branch (not officially supported by Epic).
I am surprised that you had such a bad experience with the DX10 side tho. I was working with a friend on a project around the 4.7 time (I haven’t tried it since then, so maybe its gotten worse). And we were surprised to find out that he was actually running in DX10 mode the entire length of the project coz his GPU was on the lower end of the spectrum, and we didn’t even notice because he was having very similar performance to me and the game looked identical. As for the OGL side, no idea how that looks, have had no need or intention to look at it.