I think there is no set rule but here’s a little more info to help you decide.
The main difference between Phase 3 and Phase 4 is that Phase 4 is about trying to incorporate some simpler starches and carbs again and attempt to still maintain. It’s not really a required phase – some end up finding they are pretty content to just eat P3 style in general daily life.
So with that in mind, as long as you are taking the full required break from hCG (the 6 weeks total after first round, 8 weeks total after 2nd round, 10 weeks, etc), whether you spend that entire time eating P3 style, or P3 and then trying to delve into P4, is up to you.
So example, after your 2nd round of hCG, you need to take an 8 week break in total before starting round 3.
During that 8 weeks you can either do P3 for 3 weeks and then P4 for 5 weeks, or you can just do P3 the entire 8 weeks, or you can mix it up more and do P3 for 5 weeks and spend the last 3 weeks trying out a little bit of P4.
I hope that helps!