2.
Mathematical explanation: 4/8 is equal to 2/4 which is equal to 1/2. 4 eighths is equivalent to two fourths.
Musical explanation: In music, quarter notes get a whole beat, and eighth notes get half of a beat (this is assuming the piece is written in common time, 3/4 time, or any basic time). So if there are four half beats, that is equal to two whole beats, so two quarter notes.