how to connect 4-bits D flip-flops to drive a stepper motor?

 

You need to describe your problem much more clearly (which kind of stepper, for instance) and also show YOUR efforts to work YOUR homework problem.

 

Don't know about 4 D flip-flops, but I can do a 4 phase unipolar motor with an 8-bit shift register chip and no external logic.

 

Don't forget, if you have 4 flip-flops you can have 16 possible states. In fact only 4 states are needed to drive most stepper motors. Therefore only 2 flip-flops are needed.