It heavily depends on how hard one’s work is and how much time one has. From my personal experience:
1. I used to learn English around 5-8 hours everyday by reading native books that I did not understand.
2. I studied Japanese during three days for 10 hours each day, which resulted in 300 kanji learned in total.
Taking that into account, my personal opinion is that four hours/day is not a border, and one needs to explore his limits on his own.
It heavily depends on how hard one’s work is and how much time one has. From my personal experience:
1. I used to learn English around 5-8 hours everyday by reading native books that I did not understand.
2. I studied Japanese during three days for 10 hours each day, which resulted in 300 kanji learned in total.
Taking that into account, my personal opinion is that four hours/day is not a border, and one needs to explore his limits on his own.