Even with Knights Vow, the ADC and Support would still be pretty vulnerable if they were on their own, or only had one other member.
Also, you need to consider how the enemy might defend against such a split push. A 5 man squad might push the 3 man back (and possibly overrun them) And they would be able to collapse on the 2 man lane. a 1-3-1 would have a similar problem, except that you are pushing 3 lanes and not just 2, which means the enemy would lose more resources, and if they did push the mid lane out, they would have to divide their squad to rout both split pushers.
And the 4-1, usually works because a 5v4 is still pretty hard to win decisively. (Meaning, quickly with few casualties) Also, the one member in the side lane, is usually going to be a duelist, They usually work best alone, than in a skirmish. And it usually takes more than 1 opponent to actually stop him. (With the right composition, a 4-1 can't be defended by another 4-1)