Your PSA scores since the surgery have remained very low.
After 4 years its worth getting a Pet PSMA scan which picks up any possible cells that may have cancer.
This is via absorption of isotopes which show up in a different color.
I had the prostate removed in Sept 2018 but the PSA rose rapidly and reached 5.17 recently.
I had a few sessions of radio therapy to treat lymph nodes that showed up on the scans.
Your situation looks excellent but take the Psma pet scan to be at ease