If you want to record fast action in 4K with your iPhone, the iPhone 8 and iPhone 8 Plus record more frames per second than the iPhone 7 can. This means you end up with a better video file for editing later or for showing fast motion like a race car or a kid sprinting past during soccer, football or track.
Go to Settings -> Camera -> Record Video -> choose 4K at 60FPS. This is an important setting to check even if you don't want the 60 FPS option since many people think they are recording 4K on the iPhone 8 and are still in 1080P.
The downside is that this will take up a lot of storage if you use it all the time, so keep that in mind when you turn it on. I don't use it for all of my 4K videos because it simply uses too much memory.