Wenwen's father goes to the gym every three days.

3 and 4 are prime numbers, so the least common multiple of 3 and 4 is: 3×4= 12.

So they have a rest at the same time on March 4th, so the next time they have a rest at the same time is: 4+ 12= 16, that is, April16;

So the answer is: 4 16.