If you have access to 4 (you are a ChatGPT Plus subscriber) you will have a pull-down to choose that model. One detail is that 3.5 has a green icon and 4 has a black icon. I see a lot of people online posting screenshots of their experiences thinking it's 4 but not, so that's a good way to help distinguish if someone's just confused.

While 4 is obviously a lot smarter, in a lot of cases I prefer to use the "Browsing" model - it's 3.5 but having (flaky) internet access is still a good tradeoff and I can save my 4 rate limit for more complex queries.