Unless you really have a lot of time on your hands....I would never bother to use the bus/route system from Mobay to Negril. It may be relatively cheap...but it's not convenient unless you know the system well and I'm going to assume you have luggage with you to drag along.
You can hop on a Juta shuttle at the Mobay airport for around $20us per person or why not make it even better & hire a private driver for your group of four at a cost of between $60-$80us depending on the driver you use. Way more freedom with the private transfer.
Here's a list of great drivers.... http://negril.com/transport/