I would emphasise that you can be productive for far more than 4 hours a day. For example doing routine clerical work. But there does seem to limits on work at deliberate practice level or above. If you disagree, install Anki on your phone and download or make a deck of some things you are interested in memorizing. Keep adding cards until you have done 4 hours according to Anki (clock time may be 50% longer as you goofed off at various times without realizing it). Now do this for a week and report back.
Often people will say they practice violin for 6 hours, but you will usually find that there is a lot of down time in there.
I would emphasise that you can be productive for far more than 4 hours a day. For example doing routine clerical work. But there does seem to limits on work at deliberate practice level or above. If you disagree, install Anki on your phone and download or make a deck of some things you are interested in memorizing. Keep adding cards until you have done 4 hours according to Anki (clock time may be 50% longer as you goofed off at various times without realizing it). Now do this for a week and report back.
Often people will say they practice violin for 6 hours, but you will usually find that there is a lot of down time in there.