My bitch was scanned not in pup at nearly four weeks gone. The question is should her vulva still be very very spongy, she would of been five weeks today. It is almost like elastic

I'd keep an eye on her just in case, it has been known for dogs to be scanned and told no pups for the dog to have pups.  Is she showing any other signs?

No there is no other signs at all, apart from she is going a bit baggy underneath, but she does that after every season anyway.