I believe the quick answer is No, for a number of reasons.
Note, While I believe you're talking about CSS, you don't exactly state whether you're talking about the CSS of bootstrap or the JS of bootstrap, or both.
Regardless, surely both bootstrap 3 and 4 both have styles and JS functions with the same names, and would surely conflict - or just be overwritten. (The last thing loaded usually overwrites the previous thing loaded).
Secondly, a little comment here about cooperation.
The team should standardize on what to use, not go in different directions on something like this. ("Too many chefs in the kitchen" comes to mind.)
There should be a serious talk / debate (not argument ) about the pluses and minuses of which bootstrap to use. I acknowledge, that there are perfectly valid reasons not to use Bootstrap 4 (that's another story), but honestly - just because something is the latest thing, doesn't mean you HAVE to use it, and there are many more examples and "best practices" of Bootstrap 3 out there, at this point in time.
Regardless,I'd suggest that your team stay focused on solving the current problem at hand, IMHO.
i m doing a project with a team.i made a html page using bootstrap 3 and others are using bootstrap 4
--> are you sure it's a team ? because a team is supposed to discuss and share before coding - Temani Afif