4K Stock Video too large for website

At typical 4K quality, 3MB is less than half a second of video. For 3MB I think they are expecting to use much smaller, much shorter, much lower quality. 

However, I read this: https://wordpress.com/support/videopress/recommended-video-settings/

It gives recommended video settings - you would be expected to use a video editor to reach these, not just use the production quality files downloaded from the stock site, or even files straight from a camera. The recommendations on that page, though, say 12mbps is fine for HD (a quarter of 4K). In that 3MB would be just 2 seconds of video. 3MB really isn't a video at all, it's a tiny  flash of info. Where do you see this recommendation of 3MB? I can't find anthing on WordPress giving this recommendation.

You need to downsize the video. Do you really need 4k? I would try first with HD. And then, you may also increase the compression (that will lower the quality). It is important that you find the sweetpoint for your video. That is unfortunately dependant on the video and your requirements. If yo have the Adobe media encoder (comes with Premiere Pro), you can try different settings. If not: any current video editor should be able to help you out.