Downsample in Youtube or in editing software?
If you can afford the additional upload and processing time to You Tube, you should use 4k.
Not only will this look better for most people (Not actually look better, but rather it looks more modern to an average user to have 4k and some people don't even watch it if it says FHD). But also, YouTube gives 4k videos a higher bitrate which also FHD videos profit from.

So you will get an better FHD video from downsampling on You Tube, than do it yourself.

If you don't have the time tho to wait for upload and processing, most people won't notice the difference