On some strains it is totally normal for some or even most of the fan leaves to yellow as the plant uses its own nitrogen. But I think you should have fed it more, personally. Mine are at 4 1/2 weeks and have some lower leaves drying up & falling off but none yellowing and they don't have any red hairs, only white, but like I said it may be strain dependent.

You'll need to check the color/clarity of the trichome crystals to determine when to harvest. You can get a 30X handheld microscope from eBay for about $10. Do a search on the site here and you'll find a trichome picture guide that is pretty good.

Good luck and happy growing