Warning: running duplicates of base game for ENB's
(Note: I cannot confirm if this is the same for any versions other than Steam EFLC)
when duplicating the base game, you need to modify the registry to point to your new path (i create a .reg file for each copy of the game and run it before playing)
it seems that the game (or one of the injectors) reads this value from the registry:
If it cant find the path the game crashes.
If it is pointing to a path that exists but different to the game you've loaded (eg another ENB installation), then it will load the wrong shaders (from what I can tell) for the ENB you're currently playing.
so i set it to eg: D:\Games\GTA4 EFLC\EFLC - CryENB (which is a duplicate of my Steam copy, but with CryENB installed), before i run the .exe from that folder. Everything then looks correct.
For a while I was dismissing some ENB's as not working properly, but it was actually due to the issue above, and once I fixed that they looked perfect.
I hope this helps anyone having any odd issues.