I would definitely cut down on the milk now, as they are eating the creep well.
The theory is that you can wean at about 6 weeks as long as they are eating sufficient solids. Milk. as well as being a perfect feed for young stock, is also a perfect feed for bacteria so as soon as you can get them converted 100% to creep feed, the less chance there is of stomach upsets.
The lambs of course will try and convince you otherwise and still come up pushing you and pleading for the bottle. So an abrupt weaning is often easier, although whether you wean them suddenly or gradually is up to you as the growth performance will be about the same.
I personally aim at weaning at about 6 weeks, or whenever the powdered milk runs out - whichever is the later.