i need bootstrap version choice (i'm making on-line shopping site with jsp, spring)

There are bootstrap 4.0 and bootstrap 3.x ...etc...

I'm making on-line shopping site with Java Server Page, Spring M V C) I need to choose bootstrap version 3.x or 4.0 which one is better?? Is it possible to use these versions coincidentally?? Or is there a problem when using version 4.0 now because it is new version recently.. please let me know.... please let me know.... please let me know....