On other cars I have been able to flush the brake fluid by draining as much as possible from the master brake reservoir and opening one or more bleeder valves. While this procedure is going on, keeping the reservoir filled with fresh brake fluid. Will this procedure work on a Gen 4 Prius?
Different different year but relevant https://attachments.priuschat.com/attachment-files/2018/03/143184_74070_Prius_Brake_Bleed_1.pdf
No. Prius brakes are complex. (Regenerative, Friction, Engine,etc.) Whenever the test strips indicate you need new fluid, I would take it to a hybrid mechanic. It is not a 'shade tree' system.