Witch CPU Should i get and how much ram do i need to have good 4k performance on my computer.?

Never heard of a Witch CPU.

Know what motherboard you have. Most makers of motherboards can give you a list of CPUs that will work on your board.

 Worry more about the graphics card. 4 or 500 dollars can get you a great graphics card. Your resolution capabilities are governed by what the graphics are capable of.

lo que puede ser lento es el wifi pruebe usarlo en horario menos convencional

Video stuff is determined by your GPU (Graphics Processing Unit) not your CPU. Get a fairly nice GPU like an RTX 2060 ($300) if you plan on gaming on it. It doesn't take an insane GPU to run chrome and photoshop on 4k. If you still plan on upgrading the CPU, I would recommend getting a Ryzen 9 3900X over the other options. It's performance is much better for workstation tasks than intel counterparts as it has 12 cores instead of the 8 in the i9 9900K. 

Why not add a graphics card (GPU) to the computer to take over the video load and take that work load off the CPU??? You'll spend a lot more money on an i9 system than a good graphics card will cost.

You have a very simplistic way of using whatever your trying to use.

I suggest you go ask the store clerk at the 'computer store' to recommend a add on video card with 16 or more gigs of ram.

Good video starts with your video card -- not your CPU.  

It's difficult to give you specific advice, sir, as YOU have not offered ANY useful information about your present computer system.

I agree with A.J. that it's PROBABLY more to do with your video card (GPU) that's causing your lack of performance.

A Witch will not help.

Do not buy anything until you know what you are doing. You did not state what computer you have now. A CPU must be computer motherboard compatible. The problem is most likely the graphics processor. Without knowing your computer details, that is all that can be answered.