Ive got the Chillitech bullet Mk2, I doubt mines 4k, as image quality isnt great, it copes best with daylight/sunny conditions, its a bit of waste of time at night.
its a decent enough camera for its price, really my biggest annoyances have been mounting it to handlebars, they dont really seem to have at the time a solid connector point, which maybe why the newer cameras have gone garmin mount compatible
battery life is ok, nothing special, and can be a bit hit and miss in terms of theres often very little indication on how much charge is in the battery, plus setting the time is a bit of a faff and the clock can drift
I use a Cycliq Fly 6 on the rear, doubles as a light, image quality is excellent, easy to use/setup, tells you how the battery is doing, its downsides are really its cost, and size means battery life is actually quite short, maybe 3hrs-4hrs max with the light. and alot of people had problems with them breaking due to water ingress I think. You can now buy little rubber covers for them, which seem to keep the worst of the weather off, but it blocks the speaker, so it sounds like its stuck in a bucket, but good mounting issues make it feel secure.
then finally on my n+1, Ive got a Cycliq fly 12, oddly image quality not as great as the Fly 6, but still streets ahead of the Chillitech, good battery life 6-7hrs though I dont use the light, and the original mounting wasnt strong enough, but they brought out a modified version which has been alot better.
again easy to use as the Fly 6, uses the same app on your phone to sync time. downsides again really are the cost.
I suspect when Im in the market again for another camera it will be a cycliq