I had an SJ5000 WiFi for £90 and was great until I lost it.
You have to be really carefull as there is so many fake action cameras laballed as SJ Cams.
The SJ5000 has an upgraded image sensor & adapts to lighting changes a lot quicker.
Also, the SJ4000 can not record true 60fps as its 30fps doubled... pointless.
Watch all the Vids from this guy hes a bit of an expert on chinese action cams...
http://www.techmoan.com/blog/2014/2/3/the-sj4000-action-camera-review.html