If you need 4k you’re going to need a shit ton of storage. If you go for the good quality profile 4ks you’re looking at 50GB easily per file.
Sonarr and Radarr can fetch downloads, yes. You’ll need to configure your indexers and then you’ll need to set up your download clients. I use a torrent server and sabnzbd.
You’ll need a graphics card that can handle transcoding 4k I’m not sure which is best. Ram and CPU won’t be the biggest concerns for you.