Netflix recommends 15mbps for 4K streaming so you can use that a guideline.

Unlike home wifi, where you need a higher speed to accommodate for multiple devices, with your phone you generally only have your phone using the data so a higher speed really isn’t necessary. 
The only advantage I’d see is that if you do a lot of file downloading, that would take advantage of the faster speeds. After that, 100mbps is completely fine in my opinion.

@Hairyman4u I would take more data if I were you.  100mbps speed will be more than enough, I doubt you will be able to notice the difference at all.  One caveat, if you are an avid triple A gamer, it may effect your skill. Most of us still play on a console though. 

Nobody really downloads 4K video to a phone unless they are transferring files in bulk for later use or creating a personal wifi hotspot. Most internet video sites have built-in gates to direct traffic for phone or desktop/large screen use automatically since the pricier resolutions are of little or no use on a screen the size of a typical phone. Otherwise, you would be burning thru your data for no good reason.