How often would you push someone on a swing to create a frequency of. 4 hertz

The unit "Hertz" means "per second".

Anything happening with a frequency of 0.4 Hz is happening 0.4 per second.

Technically, you must push the swing 0.4 times per second.

But since it's very hard to push 0.4 of one time, you have to be

able to figure out that you should push once every 2.5 seconds.

(1 push / 2.5 sec) = (1/2.5) push/sec = 0.4 per sec = 0.4 Hz.