OP04 is an Output Submodule with 4 Relays which plugs in to CM9001 in place of OP01 (with 8 open-collector Outputs).
This connects 4 relays direct to Outputs 1 to 4 of CM9001. Outputs 5 to 8 are not available when OP04 is plugged in
With the OPO1 if the board goes you would just replace the board, if this goes you have to replace the board or is that a resetable fuse set into it, also what what is the voltage max capacity.
If this fails you also replace the submodule just line OP03 and OP01
It is less likely to fail than OP03 or OP01 because the relays have 5000V isolation/breakdown volatge.
In OP01 and OP03 the outputs are open collector transistor outputs and are more easily damaged