At the beginning of each piece of music, after the treble clef, is the time signature.
The top number indicates the number of beats per bar (in this case four) and the bottom number indicates the type of note receiving each beat (in this case a quarter note).
Thus in * time there must be the equivalent of 4 quarter note beats per bar, e.g.
* is the most common time signature and is sometimes represented by the common time symbol.
The next most common time signature is is Three Four Time written ^.
^ (pronounced Three Four time) indicates 3 quarter note beats per bar.