This sounds like one of my girls. When 4 weeks hits...she is tired of them! (2 litters under her belt has proven this to me, hee hee) I would suggest in-closing her in a small ex-pen with them for nursing. That's what i've had to do. Some moms are great in the beginning then they just tire of them really fast. It doesnt mean they are a bad mommy, I'm not really sure what it means...
But I have been in your shoes and putting them in a small area with her at different times of the day is a good way to ensure they get to nurse. Have you ever noticed when pups get around the 4 wk. point they will actually stand under mom and nurse? If she is in-closed with them, she doesnt have much of a choice other than to let them...Hope this helps. If not, maybe there is someone else who has a different outlook/idea. best wishes,