Sorry to hear this, but if his PSA has been slowly climbing over 4 years, then hs is in a v good place to be able to make decisions in an unhurried way.
He has 4 choices - surgery (bigger risk of impotence, but often recommended by urologists in younger men), external beam radiotherapy (but tbh, as it sounds very early, this might not be the best choice unless he can get very high tech radiotherapy which protects all the other bits of him better than conventional sorts), High dose rate brachytherapy (not many UK hospitals offer this, but it is excellent) or seed brachytherapy (one day in hospital, great results).
Just depending on all the factors your dad has (psa level, the biopsy results etc) and what is important to him (for instance, some men are unworried by impotence but would do anything to avoid incontinence) he'll want to balance the benefits of the different treatments.
For a man with early stage cancer like your dad, treated with seed brachytherapy (brachy is my specialist area, so I know the UK stats pretty well), 97% of men will have absolutely no sign of any cancer after 5 years, so it is very, very likely to all be OK