You could use the latest version that is supported by GitHub Pages (right now it is 3.8.5), most plugins will work with it. Usually, it takes time for GitHub as well as for plugins authors to upgrade to the latest version of Jekyll. You will not be missing out much except for building time improvement.
You can specify which version you want to use in your Gemfile, i.e. gem "jekyll", "~> 3.8.5"
Yes, I believe so. A new install will get you on version 4.0.0. You’ll have to specify an earlier version in your Gemfile if you want to use some plugins that are not yet working with 4.0.0. (a lot of plugins already do).