Momentum is never created or destroyed. It's conserved. The total amount after a collision is the same as it was before the collision.
In this scenario, the total is 10 kg-m/s before the collision, so it's going to be 10 kg-m/s after.
Ball 2 has 4 kg-m/s, so Ball 1 has the other 6 kg-m/s of momentum.
