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You can absolutely put an ARM module (or any other module) there and just inhibit the OA4 in the configuration. Inhibiting and uninihibiting can be done online and will prevent the PLC from trying to establish a connection to that module, and it therefore won't matter what it is.
If you actually need the outputs, you could potentially get away with using an OW4 in place of an OA4 if you had electronic keying for that module set to Disable. I've never tried and I wouldn't absolutely promise it would work, but it would be worth a shot. Unfortunately I'm 99.9% certain that changing that setting from the default "compatible module" to "disable" requires a download, so it's no better than just changing it to the OW4 in the first place.