If you bought them used someone probably removed the second spring to make them easier to set. I picked up some single spring bridger #5s that way. Just replace the second spring and they are good to go.
Never understood why anyone would think a single spring would be easier to set than a double longspring. You have nothing to grab on the other side of the trap on single longspring.
I am unaware if they made single spring #4. They did make some #3 sized single springs, often called a #2 1/2 in this country (not sure if it was actually termed that or if it was just local slang). One of those rides around in the toolbox of my houndtruck. Put a bobcat in a hole and I can wire that trap to a long pole or branch, stick it down the hole and the bobcat will slap it, getting caught and then you can pull him out. That's the theory anyways, only ever actually used it once. A 25 pound bobcat on the end of a limber Cascara pole is kind of like having salmon on a light trout rod.