They can be swapped as a pair, but you might change the brake balance.
The 914/6 uses 38mm pistons vs the 914/4 33mm pistons.
The 914/6 uses the same size pads as the 914/4 does on the front, but requires the large hole for the large pin.
Naturally, the 914/6 rear caliper is larger than the 914/4 variety, but does look similar.
914/4
Click to view attachment914/6
Click to view attachmentnote the lack of a rib on the back side of the 914/6 caliper