Are there big differences in bootstrap versions?

Question

css

Such a question, how good is the compatibility of different versions of bootstrap, I saw a number of articles in which they recommend using newer versions, and they write that there is no compatibility, but I made some simple simple forms and the code given for version 3 works quite well in 4. Am I right? I understand that some
features can be added between versions, but the main classes and ideology + - are unchanged?

Answer 1

bootstrap 3 - on floats
bootstrap 4+ grid - on flexes
Making new classes for each version is a shot in the leg, since bootstrap is a business tool, and the ability not to learn new classes is a plus for developers' karma.

Answer 2

50/50
Everything is in the documentation with examples. There is nothing to study there. Opened the documentation and looked.