There is nothing in your post to say what type of shower control you are replacing and if it is an inside or outside wall or what the center to center measurement is.
Mine for instance is a cheapy thing mounted on the adjacent cabinet and is also used for filling the tub. A picture or good description would be nice, in lieu of that, there are four listed in the current catalogs for shower only, two Phoenix and two Catalina, all four have 4" centers, they run from $20 to $50. If you have a Phoenix fixture, there are repair parts listed. There are six more that are tub/shower fixtures.
I would guess that you could find fixtures at any of the big box stores for anything with wider center to center measurements but I don't know about the 4' center to center fixtures.
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