I use Filmora as well, you can export 4K all the way up to 4096x2160 use H.264 (HD Optimized) > 30FPS (as this is optimal for you tube) with a bit rate of 80000, select best for quality then export, I hope you have a very fast upload internet connection (Check it here) as a 4-5 minute 4K video takes me 2 hours to upload ( my Internet speed sucks! ) then it will take you tube 12-24 hours to distribute your video to all the global servers before it will automatically play in 4K, you can select 4K in the you tube settings, again depending on your internet speed if it is not fully distributed it will probably buffer.