My father, 77yo, has prostate cancer, mild case. His PSA was also 4. The doctor prescribed him radiation and testosterone supression injections. His PSA went to zero and his prostate shrunk. The lack of test gave him many other issues, deregulating other mild health issues he has (diabetes, thyroid, arthritis). This treatment went for 6 months.
Now his PSA, even within normal range, is getting higher (wich is normal), but the doctor wants to supress his test to zero again (sits at 100 right now). I believethe doctor want PSA near zero for 4 years, to "kill" the cancer, if ever it will happen. No radiotherapy needed by now.