I would guess you guys don't install fire alarm systems.
Many times the horn/strobes, speaker strobes and strobe only devices will only mount to a 4" square box.
We have been remodeling an existing hotel a floor at a time and that old work plate is perfect for what we are doing.
Right now we are using an accessory from steel city that makes their 4" squares 'old work' but this new item looks universal and better.
In the past we have taken the old work ears of of standard device boxes and fitted them onto 4" square boxes then use Madison bars to hold it in place.
Keep in mind this can be real fun with four MC cables attached to the box.