4 weeks for less than a pound of hay? Are you feeding it unlimited amounts? One of those bags lasts like a day for my two girls...
Yes, they get unlimited amounts 24/7 and it takes Molly, my adult pig, about 4-5 days to finish one hayrack full. I will try to put some loose in her "kitchen" to see if she will eat more that way. It is usually stuffed in her hay rack. My baby pig is only 5 weeks old so she does not eat alot yet, but also has unlimited access and gets some alfalfa mixed in.