The port configuration on the fourth unit will be blank/default when it becomes a stack member. Once you add the unit to the stack, you will have to reconfigure the ports.
For preparing the plain untagged VLAN config you could do this:
- issue "show config port" on the single unit
- copy output to a text editor
- replace "ge.1." with "ge.4."
- add unit to stack
- copy commands from text editor to CLI
Be aware that there might be other Port-based settings that you might need to configure (e.g. tagged vlan egress, LLDP, policy, authentication).