After minimizing, the equation for the D input to flip-flop A2 is A2~A1 + A2~A0 + ~A2A1A0 and the equation for the D input to flip-flop A3 is A3~A2 + A3~A1 + A3~A0 + ~A3A2A1A0
The complete diagram is below.
You may also download 4-bit_up_counter.ckt and run it in CircuitMaker. Depending on your browser, you will probably need to right-click on the link and select "Save File As..." to download the .ckt file.