Nothing will beat single board computer like rpi in power consuption, but PC can be efficient as well. For example in my country I pay around 1€ for each Watt a year (average 0.12€/kwh). So rpi is probably 5€ a year and my new server is like 20-30. There are some servers that run at 10W on idle.
Its 5x difference, but I got much better server and still acceptable cost. Having sata potrs will already save you a lot of $ and time since you need additional hardware to use ssd/hdd on rpi, especially if you need more than 1. And then rpi is not that small anymore. Rpi cant transcode at all, but its good enough for direct play. If you get intel cpu with quicksync you should be golden. I didnt test on 4k, but my g3930 can handle 4x 1080p easy.
I used rpi4 with 4gb ram as media/torrent server for few years and it was simple set and foeget setup untill SSD upgrade when it started started acting up. Figured the rpi cant provide enough juice on USB ports and then I got external power HDD case and got more into self hosting. 4GB soon became a bottleneck so I ended up with new PC and I love it. Still using rpi for 2nd pihole and few more services.