No test is perfect and the PSA (prostate specific antigen) is certainly no exception. There are no guarantees in life and you can have prostate cancer, or not, at any given level. Levels below 4 are usually considered to be reassuring and make prostate cancer less likely and levels above 10 are very concerning for suggesting a very high risk. Levels between 4 and 10 are indeterminate.
Most urologists would recommend doing an ultrasound and biopsy for a PSA of 8 but watching it to see how fast it rises would not be totally unreasonable. The risk of watching it is that it could lead to a delay in diagnosing a cancer and allowing it to be more advanced when it is found. The risks of a biopsy are complications of the procedure (such as infection), a false sense of security if it is falsely negative, and that you may end up finding and, then treating (with resultant impotence and incontinence), a slow growing tumor that never would have caused you any problems.