Scalingo https://scalingo.com is basically a french Heroku-compatible PaaS based on a reliable french IaaS (Outscale, cloud subsidiary of Dassault Systemes, the famous maker of Catia, Solidworkds, etc)
There's a few refinements in BS4. Refinements that become quickly inavouable. First and foremost the new flexbox directives all grid related stuff (mobile first). That's why I systematically migrate all my projects to BS4 even though it's not 100% needed.
This is why you should write your app code to be independant from your current provider, Always thinking that you may have to change (maybe urgently) in the future