Hello ......
I have my Oncologists explanation of a nearly identical situation.
I did not have any luck with either oral meds (see below)! In between the two oral meds, I was on taxotere for 30-weeks and it did work for me by reducing my PSA by close to 80%.
However, after Xtandi (which did not work for me) - my Onco put me on Jevtana - since it had been only about 6-months since being on Taxotere for 10-treatments.
As he said, Jevtana is very close to Taxotere in composition. I was on Jevtana for 6-treatments (every three weeks)! It worked well for me, but in order to slightly reduce the SE's, he said he was putting me onto four week intervals with Jevtana - and called it maintenance.
The purpose of extending three weeks to four weeks, in his opinion, is to slightly reduce the SE's - which can get to be pretty strong on Jevtana - even more than on Taxotere. The first 7-treatments of Jevtana did reduce my PSA by about 82% ... but the 8th treatment barely made a difference in either the PSA or catscan.
However, his current guideline is when my PSA doubles from the current 15.3 - he will likely put me bacck on the Jevtana and for 4-week intervals.
Summary .... try not to read too much into the 4-week intervals. I do believe the purpose is to slightly reduce the wear'n tear on the body - by the chemo.
Wish you the best for your Dad.
Rob & Gizmo