Hi mate, i've just listed my hero 4 silver on eBay. its got some light damage, but functions perfectly. it'll be a cheaper option and it does film at 30fps in 4k:
https://gopro.com/help/articles/block/Available-Video-Resolutions-for-HERO4-Black-and-Silver
However, i've always found shooting at 120fps in 1080 is great quality and super smooth.
https://www.ebay.co.uk/itm/153573488390