You'll end up with much more troubles with the old HD 4xx series in CF, AMD supported different models from the same series to be crossfired well beginning with the HD 5k series.
Anyway you'll end up like getting 3 HD 4870s in Tri-Fire. The HD 4890 supposed to be syncronized with the lower clock speeds from the HD 4870.
Make sure your PCIE slots are rated at X16/X8, if one's rated at X4 you'll be bottlenecked.