My 4 month old has had chest cogestion for over a month now. How long does it usually take to go away and what's the best way to help her?

Congestion. A 4 mo is a nasal breather that often sounds quite congested. You can feel transmitted vibration on the chest wall when you hold them. Like the vibration of a clarinet reed in the mouthpiece, the sound comes out the other end. In the same way nasal congestion vibrates the chest. This subsides some when the kid switches to mouth breathing after 8-9m or if the nose is kept clean with saline.
Prolonged congestion. Congestion that has lasted over a month deserves a work up and possibly a chest x-ray. See your pediatrician post haste.