A couple of your plants it taking a bit of a beating with the fan blowing so hard on it.
You can guess by the trichs color all milky means there ready albeit some like myself like to wait till there is a bit of amber
Tbh once you get used to it you can normally tell by eye
Btw as for four weeks to go that’s not written in stone with breeders data you normally add two weeks sometimes longer either way thier advertised dates is to be taken with a pinch of salt
A good time to really start checking trichromes is when about 80 percent of the hairs have changed color and curled up. These are also in week 4. You can see a big difference between the 50/50 hybrid to the 20 indica/80 sativa. Everything will be different in harvest time. Your doing fine. Just pay attention and when you have an issue dive deep into and learn.