Just make sure it’s a genuine SJcam 4000 – there are loads of eBay adverts that list sj4000 in the title. It aren’t actually genuine. All the genuine ones (unless really old) actually hace sjcam written on the front of the camera.
I’ve got an sjcam m10 – its effectively an sj4000 in a go pro session shaped case. Works ok – compared to the basic go pro hero there isn’t much in it. The sjcam has a sharper picture, but the GoPro has more vivid colours and responds better to lie light conditions (like in thick tree cover).
Mine cost £37 delivered from Hong Kong. It’s the non wifi model. I did break the on button dropping it in its case on to concrete, but think it was a freak landing on just the wrong point. Cost me £10 to get fixed but took about a month as it went back to China.