According to Wiki, a cadence is any sequence of 2 or more chords that end a phrase, section or piece of music.
So I,VI,IV,V (or 1,6,4,5) qualifies as it is a musical phrase, ending on the 5. It is technically an "imperfect", "half" or "open" cadence.
The use of the minor on the 4 chord gives it a bit of added colour and emphasis because it contains the note Ab which is not in the key of C major.
When I played this to myself just now it seemed to me to strengthen the final (V) chord. I think this is because the minor chord on IV seems to want to lead to the V chord more strongly that the major .