You don't have to "tinker" with the action mechanism on any gun for the pull to change. Tne "new" gun I am sure came with a 4 lbs pull. The sear, hammer hooks with use will wear and angles change with use. The main spring can loose some tension with use, the sear spring can loose some tension, any number of things can take place. You hear people complain about heavy trigger pulls and roughness on new stock guns, and the well meaning advice, shoot it a while and it will break in and smooth out and lighten up. To a point this will happen. Your gun already tuned to 4 lbs so it is very possible one of, or all things took place causing a lighter pull. Just my opinion.
if you can get your hands on another set of weights or have someone else check and weigh the trigger, preferably the dept. armorer. Where you position the trigger on the bar can effect what it will pick up or not pick up in some cases.
Hope this is of some help.