So, technically, you're looking at a few things here ...
Like it's been mentioned, look at your analytics. But also, look at your render settings. One consideration with 2K/4K is the bitrate. YouTube will use a higher bitrate for higher res footage, so there is a chance that you get a bitrate, and therefore quality, bump by using 4K over 1080, even when viewing in 1080.
Ideally if you render your 1080 footage in a high bitrate and YouTube is true to it, there should be little difference in the 1080 and 4K footage when viewed at 1080. But if you upload 4K and YouTube downsamples to 1080, 720, etc, the compressor is working with a ton more pixels to create the downsampled footage. More data to start with makes for a truer downsampled video in terms of quality. So, uploading 1080 should give a good 1080 video, but a 4K downsampled to 720 will likely give a higher quality 720 than a 1080 video would once downsampled.
Clear as mud? Good.
Now, from a sub standpoint, when you upload 4K you give the impression of higher end content, even if you're just starting out. A 4K or HD stamp on your thumbnail matters to viewers, even if it doesn't show in the quality of the video itself. It's a mind game. "Oh, this dude shoots in 4K ... he must be legit."
So based on that, yeah, do 4K. It's hard for me to accept this reality myself, as I know that a 1080p60 or 1080p48 video will look better on most devices than a 4K24 or 4K30 video will when it comes to a motovlog.