Crossfire is pretty lenient on performance loss from 8x to 4x. you will lose at most like maybe 4-6% compared to an 8x/8x doing the same.
i will also note that you may experience microstuttering if your eyesight is very sharp on framerate reading.
edit2: when i say 4-6%, thats the most loss upper limit. its probably going to be less, as early gpus dont saturate the slot as much as like extremely high end ones.