Stoeger 22 cal. lugers
 

What is the difference between the models STLR-4 and the STLR-5?

4 1/2" bbl. vs. 5 1/2" bbl.

They were also made in forged alloy and cast steel frame variations. I don't know if the latter had different model numbers, but you can easily tell the rarer steel ones by the red and green holes in the safety area, opposed to the raised "F" and "S" of the alloy frames.