How to solve 70-200 f4 is focus slipping issue?

I think I know what the cause of that issue is, but first I'll have to briefly explain how the focus assembly. The focus assembly is comprised of three rings:

• The autofocus ring, which the lens turns using the USM motor.
• The manual focus ring, which is what turns as you turn the rubber focus ring on the camera
• The main focus ring, which is sandwiched between the two other rings and is what actually focuses the lens.

These three rings need to be held together with enough tension for the autofocus and manual focus rings to be able to turn the main focus ring.

What's appearing to happen with your lens is that there is insufficient tension, causing the manual focus ring to struggle to turn the main focus ring. The insufficient tension would be caused by either a loose retention ring or a worn down wave spring. Both of these fixes will require you disassemble the lens and remove the focus assembly. Here is a guide I found for your lens which you can follow along with until you get to the point of the focus assembly being removed.

The retention ring (as pointed out in the image above) is a flat metal ring that holds all the components on the focus assembly together. This ring will have threadlocker on the top of it to prevent the ring from coming loose. However, overtime the rings can still sometimes loosen up. If this is the case, all you need to do is rotate the ring as far clockwise as it'll go. Then place on additional threadlocker to keep it held in place. Follow this guide for reassembly of the lens.

If you look at the image above, you can see a thin single-turn wave spring directly below the retention ring. Overtime this spring can wear out and cause reduced tension in the focus assembly. To get to the wave spring, you first need to push down slightly on the retention ring and rotating it as far counter-clockwise as it will let you. You should then be able to just lift the retention ring right off. Note that there will likely be threadlocker holding the retention ring in place. With that off, you can pull of the wave spring.

If you want to salvage the spring, you can try using your fingers to make the waves on the spring more pronounced, which should hopefully allow for it to create more tension. You'd want to be careful about adjusting the spring too much, as it might be possible for there to be too much tension (I haven't personally done this repair, so I don't have personal experience with this). Alternatively, you could try to find a replacement spring, but they could be hard to find since the focus assembly isn't considered to be "serviceable". Your best bet to find a replacement spring would be just removing one off of a different broken focus assembly.

Once the spring is fixed or replaced, you can place the spring back in place. Next place back the retention ring (you will need to rotate the ring until the tab line up to allow it to slide on) and rotate it all the way clockwise as it'll go. Then place on additional threadlocker to keep it held in place. Follow this guide for reassembly of the lens.