How to share a VCC and a GND across to multiple relays
When we look at the internal circuit diagram of the relay it looks like the following:



I have also added one motor to show how the connections are to be made.

Vcc is the signal given by the microcontroller (one of the output pins from the Arduino). When this signal is given the SPDT switch changes state to NO. This completes the circuit and the motor will rotate.

Note Vcc is just a 5v signal if from the microcontroller (Assuming this is the rating of the relay). But Bat2 needs so supply the required voltage and current for the motor to run.



Now moving onto what you want to achieve in your project, the description is vague. You are running 2 pairs of motors with 4 Relays. I presume each pair is controlled by 2 switches. One for moving forward and one for moving backward. The same for the other pair.

The circuit diagram looks like the following:





Truth Table:
U1 - M1 and M2 Forward

U2 - M1 and M2 Backward

U3 - M3 and M4 Forward

U4 - M3 and M4 Bakcward

U1 and U2 - Nothing

U3 and U4 - Nothing

All other combinations can be inferred as M3 and M4 are isolated from M1 and M2. You have to make sure your battery can handle running all 4 motors at once though.