I was told I needed about 4 root canal sessions, but it stopped hurting after the 2nd visit. Can I skip the rest?

Even if the pain is gone, you should still complete all the scheduled visits. The pain disappeared because the inflamed nerve tissue has been removed - it does not mean that all the bacteria inside the canals are gone. If you stop here, the remaining bacteria can multiply again and cause the inflammation to return. If the temporary filling cracks and reinfection occurs, the next round of treatment becomes much harder than the first. The canals need to be fully disinfected and sealed, and the tooth needs to be finished with a crown for it to last - so please continue with the remaining 2 visits.