A time signature of 4/4 means count 4 (top number) quarter notes (bottom number) to each bar. So the pulse, or beat, is counted 1, 2, 3, 4, 1, 2, 3, 4, and so on.

That means all the notes in each bar must add up to 4 quarter notes. Any combination of rhythms can be used as long as they add up to 4 quarter notes.