First, if you are still in a permanent teaching role, don't reduce to 4 days without getting financial advice (through your union if you are an NEU / ASCL / NAHT member) - if you struggle to work during the chemo, your full sick pay at 5 days is more valuable than your sick pay at 4 days. Also, if the oncologist determines that you have advanced prostate cancer (which seems to be where you are heading if they decide RT / focal therapy is not worthwhile), you may be eligible for health related early retirement, the enhancement on which may be 3 or 4 years' service. Another thing to take into account ( again, a financial adviser or actuary is needed) is whether you should take the pension or maximise the lump sum since you are less likely to need a pension for 30 years - our financial adviser worked out that if John took the bigger lump sum now, he could live for 17 years (until he was 76) before he started to make a loss. Since he is less likely to live to be a really old man, it was worth the risk for us.
That is all about future options though. In terms of the chemo, we do have members who have worked full time throughout their chemo - Leila's husband, I think, among others. Although it does seem to be one of the more manageable chemos, you don't know until you start how much it will affect you. My worry for you would be being around kids with all their bugs & germs - it is essential to avoid exposing yourself to injections as much as you can, particularly on days 5 - 8. Since docetaxel is usually a 3 week cycle, that might mean being off work one week in every three for a term?
If / when you get to the stage of talking about chemo, you could ask about having apalutimide or enzalutimide instead. These would be less likely to impede you from working although you would still need to be vigilant about any signs of infection / high temperature.