Well, technically, I believe that for Hill his first year was also a medical redshirt. I believe in 2018, prior to the season, he took a pitch off the wrist or hand and it messed up his hand and he had to have surgery. Problem wasn't recognized soon enough and he played a bit before being shut down for the season.
So, if he has to shut it down this season, and he may very well have to, then it would be for another medical reason. There is such thing as being granted a 6th year of eligibility, but they are fairly rare. I doubt that Hill would want to stick around for a 6th long and would be looking to go play professional baseball if he could.
In many ways, every year past your third year in a program can hurt your value as a professional, which can really derail a career. MLB teams favor their valuable picks over those that they didn't pay as much for, so the late round guys will not get as many opportunities as the early round picks that they paid a lot of money for.