I am replacing a bathroom fan/light until that operates on separate switches.

When I took out the old unit out I failed to notice how it was wired. The power line to the unit has 4 wires, 1 Black, 1 red, 1 white, and the ground. The replacement unit has 3 wires 1 black, 1 white and 1 blue. Can anyone help on how to connect the wires. Thanks

Most likely you would use this hookup:
Black to Black (probably the hot for the fan)
Red to Blue (probably the hot for the light)
White to White (neutral)
Ground to a green screw (or any screw)

Hope this helps,
WallyH