Growone makes a good point - it is alot easier to separate seeds out when they're fully formed - those little immature seeds can be a right pain in the ass as you're constantly having to crush every little ball shaped thing between ur fingers to see is it abit of ball-shaped bud or is it a tiny seed?!! Lol!
And tho i've never grown plants specifically to harvest seed, i would guess that u still need to flower for the normal 8 - 9 weeks, etc, so as to ensure what bud is on there still reaches its mature, good harvestable levels of THC & CBD, etc. To my mind, cutting a plant 3 to 4 weeks early just cause its seeded would still only result in giving bud of a quality equal to chopping a non-seeded plant 3 to 4 weeks early; i.e. not as strong or as tasty as it could've been. I know when they get seeded that plants put most of their energy into making seed, but i'm pretty certain they still ripen & finish off their flowers as well (if i remember rightly its cause they still wanna attract as much pollen as possible before season ends). I would agree with the others here who say leave it to finish as normal, but i may well be wrong & would be interested to hear from anyone who does know more too!