Personal portfolio navbar isn't working?

So I’ve been trying for a couple days to learn about navbar and how they work because they always come out bugged for me. Even if I copy and paste the examples from the bootstrap website, navbar isn’t showing and I’m just wondering what I’m doing wrong? Is there anything online that I should be looking up specifically?

Bootstrap is already added to the pen but I didn’t know I needed to add jQuery as well.

Edit: So I just switched from bootstrap 4 to bootstrap 3 and everything is showing now. I’m guessing they’re written differently? Should I be writing my code for bootstrap 4?