Should I merge my 4 star Robins first or should I unlock their potential?

The merged stats won't carry over.
You'd see it as 5star (insert level)+nothing rather than (insert level)+1.

You loose all merging bonuses when you unlock potential, therefore its recommended to only merge 5 star units.

In your specific case, just unlock the potential of the better stated Robin. Since merging them then unlocking potential right away is the same as just sending one of them home.