refers to two things, in my experience.
1. a rhythm pattern where the percussive bass takes place on every downbeat. Common in house, trance, other forms of techno, disco, etc.
it is the opposite of a breakbeat, where the percussive bass can be anywhere, but is normally not on beats 2 and 4, and rarely on 3.
2. refers to a 4 speed transmittion with the shifter on the floor (as opposed to the dash or the column)
1. I dig that four-on-the-floor beat; it goes well with the bassline.
2. John wrecked the four-on-the-floor transmittion in his car by going 80 MPH and then quickly shifting into reverse.
n. Indicates that a car has a four-speed transmission, where the shifter is "on the floor" rather than on the steering column.
That old camaro had a 454 with four on the floor.
a sex act in which one of the participants is on the floor with both arms and legs supporting them.
It was hot as hell...I was standing above her naked body and I had her from behind and she had four on the floor.