Don't worry about the number of water inlets, and don't be confused by the problem. The easiest way:

"It takes 4 hours to open three water inlet pipes" is understood as "it takes 4 hours to open the water inlet pipes"

Then it takes 8 hours to drain a pool of water when the water goes in and out together.

The question "If you open two water inlet pipes and drainage holes, three water inlet pipes and drainage holes, four water inlet pipes and drainage holes 1 hour" is understood as "How many water inlet pipes are opened to drain three quarters of the water in a drainage pool in three hours?" Just now, we already know that it takes eight hours to drain the whole pool by opening the three water inlet pipes and the drainage holes together, so three quarters.