So you want to combine 2 stacks of 3750x's together right? So Stack1 has two switches and Stack2 has two switches?
Since the software is the same on each node that is great.
One of these stacks is going to stay the same.
If it were me, I would do the following.
Which ever one you want master.
switch 1 priority 15
switch 2 priority 14
switch 3 priority 13
Since you have entered the provisioning commands, you can go ahead and put the commands on the ports, even if the switch is not currently connected.. That way when you connect the switch, it's already ready to go.
When you get ready to add SW3 to SW2, you will connect the stacking cables, and THEN power on the switch, and verify it's connected properly.
Then you will need to do the same to SW3 and SW4.
The new configuration, should be pushed to the switch, but I would erase the switch first if it were me.