What is the velocity of the first ball after the collision?

Conservation of momentum.

Assuming there was no spin (English) on the first ball when it collided with the other two:

The vectors of all three balls after the collision must add up to the momentum vector of the first ball before the collision.

Note that balls 2 and 3 are moving perpendicularly to each other.

The easiest way to solve this problem is to put the vectors of balls 2 and 3 nose to tail.
Draw a broken line vector of 4 m/s from the collision point in the same direction that ball 1 was initially traveling.

The nose of your broken line vector is where the vector for ball 1 needs to end up after you place it's tail on the nose of the resultant of vectors 2 + 3.

This method gives you the starting point (resultant of 2 + 3) and ending point (nose of broken line vector) for the new velocity vector of ball 1.