shes about 4 years old and she started lactating about 4 months ago. shes not pregnant but her nipples are starting to hang down more then usual and shes lactating.. anyone know what it could be?

Sounds like false pregnancy or an underlying medical condition.

Taking her to the vet is best.

Make her a vet appointment. Lactating for four months is not normal. This could be a major medical problem.
I definitely recommend spaying her as well. There's no need in keeping her intact- especially if she has a medical problem. Even if this were a false pregnancy (doubtful, IMO- four months would be too long for that)- false pregnancies can signify a health problem.

Yes, take dog to vet. Could be hormone imbalance and the vet might ask you about your commitment to breed or spay.
Hormone imbalance is common. Hopefully it is nothing more serious.

Breast cancer could also cause a dog to lactate. Hope you've gotten her to the vet by now.