already yellow leaves

Hey! Yeah it is a bit early.. it's most likely a potassium deficiency and it's sucking it out of your leaves.. As for 6 week flower finisher.. Nope, not nearly ready. Your in week 4 and you still have a lot of white straight pistils.. it'll take more time to finish. Harvest window opens when you start getting amber pistils(hairs). Normally when there's 70% amber and 30% white pistils they're ready to start flushing but that's not always the case. The best way to check is looking at the trichomes. Clear trichomes haven't reached their full potency, cloudy ones are full of THC and amber are losing THC while gaining CBN. So depending on your preference whether you want more of a head high or body high. I normally start to look at trichomes when there are about 50/50% white and amber pistils. I harvest when after examining and finding 70% cloudy trichomes and 30% amber trichomes. For me that's a prefect combination, depending on the strain as well. With others harvest with 10% amber trichomes.. really depends on strain and your desired high. Good luck and happy Growing!