granted the roots never truely STOP growing however they do slow down quite alot, as the roots spread out the plant gets larger, but then in flower the plant isnt really growing anymore in a vegatative way, aka getting larger and fuller yet not flowering, when its attempting to get as large as it can before the light schedule changes the roots will seek out more room, but when in flower especially after the 2 week stretch the roots and plant arnt really growing in the same way sure it is growing but not for size its trying to reproduce now, so its roots arnt searching as much and instead bulking up (those fuzzy white root strands comming off the roots) so it can absorb as much as it can from the area it occupies, if it kept searching as it did in veg it would use more of its energy for rooting rather then flowering, and as we have all seen that is not the case. so granted the roots are still growing but not in the same way, no more stretching out looking for more room, instead it settles in for the home stretch and really hunkers down, fuzzing up those roots and concentrating mainly on trying to reproduce.
also i agree that it could be a combo of being root bound and nute def but then again it is 4 weeks+ in flower and it looks pretty typical, up the feeding slightly like ozzy said and all should be good, as long as it isnt the sugar leaves and bud its self yellowing like mad your golden...especially since the yellowing seems to be below the main canopy, if the plant knows that leaf is not being used to its full compasity it gets rid of the useless leaf. as long as it isnt spreading again your golden. she may simply just be a heavy feeder.