Correct for what? What are you wanting to clean?
Donte explained things really good earlier in the thread. Use whatever tip is going to give you the pressure you want according to the nozzle chart. Just don’t ever use a nozzle that is going to restrict your 4 gpm pump. You’re going to want to learn how to read a nozzle chart.
Let’s say you wanted to wash something using 1000 psi. Look at a nozzle chart and follow to the right until you find 1000 psi. Now go down until you come to 4 gpm (your machine gpm). Now go all the way to the left to the nozzle size. So, a #8 will give you 1000 psi.
You asked about the 3.5 nozzles. Look at the nozzle chart and run it in reverse from what we just did above. This time you’re going to follow it to your pump’s max psi. What do you think you get?