4/4 has four beats in each bar, and each beat is a quarter note (that is what the lower number means). In practice it is felt as four even beats. You will hear it in almost all pop, rock, blues, funk, hip-hop and film songs. Keherwa, the most common Indian light-music taal with eight matras, fits two bars of 4/4.
The metronome above is set to 4/4: an accent on beat 1. The tempo counts quarter notes. Tap any beat to change its accent.