If you look into nav bar documentations of boot strap 4 you will notice the responsive nav bar with hamburger menu like we used to have in boot strap 3 is missing.
The nav bars in version 4 are having different UX. In my opinion, there are so many sites based on boot strap 3 navbar and people get used to the way it is working.
I personally would prefer to stick with what people are used to than introducing new way of nav bar.
Here is the sample that you need to follow