This article covers a bunch of stuff you'll need to know and calculate for this. I think your 4 6x6 idea may fit with this

https://www.ziplinegear.com/pages/how-to-build-a-zip-line-section-one-concepts

I believe you'll need to figure in a lot more wood than you may be thinking. It will probably need a platform at one or both ends, and guy anchor posts at each end.