Hello @Eleftheria25 ,
Your strategy is correct, and it is actually very easy to do. Go to Calendar, then Pricing (where you set nightly price), and scroll down to additional charges. There, you will find an option for an extra guest fee, indicating how much for each one and after how many guests to start adding the additional fee.
For a 4-guest property, I would set the price for 2 guests and then charge for the third and beyond.