There is a huge water tank and there are four pipes that can fill the water tank.

The first pipe can fill the water tank completely in exact 2 days.

The first pipe can fill the water tank completely in exact 3 days.

The first pipe can fill the water tank completely in exact 4 days.

The first pipe can fill the water tank completely in exact 6 hours

Let's say we opened all the pipes simultaneously, how long will it take the water tank to be filled completely?