Explanation of if the cell undergoes mitosis every 6 hours,how many cells will there be in all after 4 hours
If this is just a way of saying that the population doubles every 6 hours, then in 4 hours, the population will be
2^(4/6) = 1.587
times the initial population.
If you started with a single cell, of course, after 4 hours it will not yet have divided, so there will still be a single cell.