Yes, you can use a Pi4 to accomplish the results of this guide, I used a Pi3B+ for a few years without any major issues. However, you will not be able to follow this guide to get it set up, as Pi's are a different architecture and so you need different images for the initial setup regarding Ubuntu. Mostly everything after that will be the same though.
Just keep some spare copies of your setup mirror imaged to another SD card once you're all done and you are golden. Configure your download settings in Sonarr/Radarr to avoid 4k content, that's the only real limitation of the Pi's, outside of the SD card lifespan (solved mostly by just not logging).
@spacecowboy - not being able to write an image will make the Pi4 as a server a biiit more difficult. Do you have an android phone? There's etchdroid or Pi SD Card imager, which materials to use for can cost under $10 (you'd want the SD card reader that can plug into your phones port, for example). It's fleeting otherwise, chances are high that you will get it set up and then the SD card will die and you'd be out of luck regardless.. If the Pi is your only computer for now, then I'd keep it that way. Either way, I do highly suggest some backup SD cards, they are cheap and you rarely need more than 32gb for the operating system and basic usage - anything with heavy logging or storage should be kept on an external hard drive.
While it's possible with an android device, even maybe a library computer with permission for USB devices and temporary downloads would be a good option. It's really nice to be able to get your server all setup and then make a duplicate of the SD card, which I don't believe is possible on android. It's imperative to have a backup since SD cards do have a lifespan, using it as a main server with no backups is putting all your eggs in one basket. All it takes is forgetting to disable logging and the clock starts ticking.
It's also nice to be able to test out different operating systems, as you might find that Buster has more overhead than something like DietPi, a command line based OS, as well as being slightly less straightforward for your needs if the Pi is going to be a headless server. But like I said, if you're using the Pi as a regular computer, DietPi won't be a viable option since it has no GUI.