This is a test of commonsense with single/multi-choices. Complete the next sentence:

How to install sliding closet doors
Remove any existing doors on your closet.
Use a drill to unscrew any hinges or tracks from your closet opening, and take off the old doors and set them aside. When you're finished, the opening to your closet should be free of any obstructions.

Select from the following. 1. Remove any old knobs on the doors to make sure they're still functional. Knobs can weaken if you screw them back on. 2. On some models, the door may also have bolts or loops on it that allow you to turn the door clockwise or counterclockwise by hand. Measure the width of your closet doors. 3. Slide a door back in if needed until you're able to open it. Unlock the lock or flip the switch on the door. 4. Measure the height of your closet opening. Use a tape measure to measure the distance between the floor and the top of your closet opening to get the height.
The answer is:
4.