What happens if you get the red and black wires of a four prong dryer cord backwards?
This question assumes:
- we are talking about U.S. Residential wiring
- the person doing the wiring gets the green and white wires in the right place
Assuming you are asking about US residential biphase 240 volt wiring (or equivelant), then there is nothing wrong with reversing the black and red wires.
To clarify, there is really no such thing as them being backwards. Red/black, black/red, X/Y, black/black; it's all the same thing.
This is why you'll never see them identified by color. Typically in a 4-wire installation like this (NEMA 14-30 in this case) the two hots will be identified simply as X & Y.