Four trains coupling - find speed of resulting train

[dsptl]

Three identical train cars, coupled together, are rolling east at 4 m/s. A fourth car traveling east at 6 m/s catches up with the three and couples to make a four-car train. What is the speed of the four-car train?

I have no idea what to do. Please Help...thanks in advance :)

 

[Topher925]

Start by assuming a perfectly elastic collision(no energy loss in the collision). Since the collision is elastic you can assume conservation of momentum before and after the collision. What is the momentum before and after the collision?