4-Day:  
-On the second day you climb nearly 1200m (about 4000 ft) in the morning.
-On the fourth day, you reach Machu Picchu in the late morning.  You will have time to either explore the site or partake in a guided tour, depending on the particular operator.
-A good option for the athletic traveler and/or those with limited time.

5-Day:
-More time spent trekking, which means that it's a more relaxed pace and you get more time to appreciate the interesting ruins, scenery, flora, and fauna along the way.  
-Groups have time to reach often more beautiful campsites further away from the actual trail.
-You reach Machu Picchu late in the afternoon on day-four and overnight in Aguas Calientes so the majority of the fifth day is spent at Machu Picchu before taking the late afternoon train back to Cusco.