It all depends on your situation. If I want to make money and put food on the table then I stick with Bootstrap 3 because I have developed already existing components around it. If I want to upgrade to Bootstrap 4 then I risk significant productivity decrease and therefore business for the sake of some benefits that in the end have no significant functional impact for the client. So the question is are you in the middle of development. If you are, then it would be crazy to simply scrap everything. If on the other hand you are beginning development of a new app, then you should consider a transition, if it's not too much trouble. Upgrading the framework itself is not the problem, the problem is if you have tried and tested complex components built around the framework that you're wiling to sacrifice. Just cuz there is "excitement" and more positives around something new, doesn't mean you should go forth with it. You have to assess the drawbacks as well on a much broader level. Having said that, it's probably not that difficult to go from BS3 to BS4 anyhow. My point here is that listening to what the crowd sayz is best for you may not be best for you at all.