That's complete and utter fantasy, there is no need for 4tph. Henley is quite a small town
It's a small town, but there is also a lot of school traffic heading for Piggotts, and Twyford is rapidly growing too.
It would be quite a low cost conversion too
It would be better to put the loop in the Wargrave area and have it as a dynamic loop, as it would be halfway through the running time, making it easier to path. It would be really hard to get an attractive 4tph with two bays and Henley and Twyford, as they would have to follow one another on the single line. The units arrive at Henley at XX:27 & XX:57 and depart at XX:00 & XX:30, and arrive at Twyford at XX:12 & XX:42 and depart at XX:15 & XX:45.
Wargrave is a much more expensive location to place a loop in though.
The site of a second Henley line platform at Twyford is already mostly within the railway boundary, and the reinstatement of a second platform at Henley would require minimal encroachment on the adjacent car park.
Wargrave station is sandwiched closely between the Sheeplands Farm fruit polytunnels and expensive residential property, and the line is consistently close to businesses and residential property as far north as Lashbrook House north of the Thames crossing in Shiplake, and pretty much the entire way south into Twyford.
The cheapest option would probably be to get a Turbo or Sprinter retrofitted with a tube interior to add to the train on the Regatta weekend.
Find the least rusted out Class 150?
3 car Sprinter sandwiched between two two car Turbos so no need for traction knowledge
That would be very risky. If your Sprinter decides to fail on the branch, it would be a long wait to rescue it, and it would completely screw service until it could be recovered. Turbos generally have a better record.