Color change.

dude I'm the same way... every little spec or blemish.. I'm like OMG What is that.. okay first things first, it's looking great. The yellowing at the bottom is what you wanna see.. But the browning seems extremely orientated to one area.. And it's near the top upper parts which is not what you would expect this early. Too me it looks almost like wind damage.. I could be wrong but is there a static fan ?

I wouldn't think about flushing yet, the flushing begins when you think the buds are nearly ready, the harvest window is 2 weeks long and this is when the trichomes start to ripen. So it could be completely natural or out could be asking for some pk boost. 👍🌱

I would have to say you have a good 3-4 weeks left. Flowering phase is usually a good 65-70 days (8-10 weeks) depending on the strain and you're only in the 4th week of flower. You definitely want a lot more of the white pistils to change color and turn inward toward the calyx. The last week or so, I would stop feeding and use water only for the rest of the grow. Also, harvest time really depends on the trichome color, so make sure to get a good magnifying glass or jewelers loupe so you can get a close-up view of the trichomes. At the very least, you want the trichomes to appear completely white/cloudy without no clear areas. I prefer to harvest when the trichomes on the buds (not the leaves) are completely milky with about 20% amber. That gives a great stone and not too much couch lock. I hope this helps!