I agree that you needd to initially keep both pairs seperate but start scent swapping quickly with bedding and stuff.
My thought and thats all it is, is to introduce both pairs as pairs and not individually as they then have a friend to support them. Make sure they all have a place to run and hide if they want.
I had two pairs that lived together for about 5 years with eventually indifference to the other pair, the start was terrible though.
You dont say what age the cats are cos that may make a difference.