Sorry, 4 hours is too much for me, and for most tennis players.
Would you believe? Practice is harder than matches! Matches, your adrenalin is up, your interest is up, you've prepared for this moment.
I've hung around practices for CCSF and SFSU, and while TOTAL time of practice might go longer than 3 hours, the player's actual HITTING and movement practice times were closer to 2/3rds or half that.
I've sat in the boat while SammyDuval was practicing. He never did more than 10 slalom passes, or half that many jump practices in one day.
RonGrant and ArtBauman were friends of mine. They only took enough laps to familiarize the track, and had other rider's, including me, break in the various sets of tires.
I went from Novice to 125/Open Pro in motocross in 6 years. Never have I seen any rider practice longer than 45 minutes. Sometimes, they'll ride the bike longer, but it's not practice, and they're not pushing. I used to give rides to RodneySmith, bringing him from his home to our practice track behind Shell's Oil Refinery in Concord/Martinez.
For practice, most top performers warm up/stretch first, then go moderate intensity for a few minutes, then go full speed for less than an hour, because concentration don't last much longer, then cool down and cruise a few, adding the FUN FACTOR back into the equation.
I seriously doubt even a Nadal would practice more than 2 hours at a time, twice a day, during the off season. Nobody works harder than Nadal.