Not too sure, everyone may be right here this does look like the plant is comming to an end but thats way too early at 4 weeks this plant finishes at 9 weeks all though 8 is better for THC (slightly lower yield) The size of the buds in the picture are deceiving due to the large hairs, have any of these started to turn brown? Doesnt like like it.
I aint no expert on this plant but think it may be inconsistant levels of nute.
Dont think you need to worry too much though as whatever your doing seams to be working, buds look nice. and I think this plant looses a lot of it's leaves towards the end but that may just be the ones ive seen.
Post some more pics as it goes on mate.
Sk Dr