Do you mean 2 x 4ohm speakers on (say) the left channel and 2 x 8 ohm speakers on the right?
2 x 4 ohm cabinets in parallell means the amp will "see" 2 ohms. A pair of 8 ohm cabinets in parallell will be "seen" as 4 ohms by the amp.
If your amp is designed to drive a 2 ohm load it will run OK but the 2 ohm side will have a greater output than the 4 ohm side.
More to the point, why do you want to do it?
By the way, an 8 plus a 4 will produce 6 ohms. Much easier for your amp to handle if it's OK handling 8 or 4 ohms per side.