What is the final velocity of a runner who is accelerating at 2 feet per second squared for 3 second with an initial velocity of 4 feet per second

He starts at a speed of 4 fps.

He adds 2 fps of speed during each of 3 seconds, for a gain of 6 fps.

At the end of the 3 sec, his speed is (4 + 6) = 10 fps.

We know nothing about the direction in which he runs, so we're in no position to discuss velocity. The best we can do is to determine his final speed.