therock,
Unfortunately it is the worst time to purchase new Video cards, because of demand prices are way over retail and difficult to find , Nvidia and AMD
Can you define streaming 4K video, do you mean stream 4K movies from Netfliks, Amazon, or is he going to stream the games he is playing for others to watch, big difference
Pretty much any GTX 1030 or higher is going to stream movies from Netflix at 4K without issue, the on die GPU of the latest processors can do that.
If he is going to play newer games at 4K a GTX 1080 is minimum with detail turned down, a GTX 1080ti for 4K with graphics details set high
As for monitors if you want to stream Netflix or Amazon Prime at 4K you will need either a 4K UHD TV or a Monitor that is HDCP 2.2. compliant, not all Video cards and less monitors meet this requirement, and some TV's make good monitors , both Video card and Monitor have to meet the correct specifications
What OS is he using Win 7, Win 10, there may be a requirement for Win 10 but I am not sure
Please research (google) the requirements to make sure your not purchasing hardware that does not meet the requirements