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
After leaving the Ubuntu world a few releases ago because I was tired of having old Gnome releases and because I wanted Wayland because of its Hidpi support, I will surely come back :)