You will need at least 4 people for boosting.
Settings should be:
Time: 60 minutes
The infected has to run to the human spawn. When he has killed an orange guy it is faster to grab a shotgun for the spawn kills. The 3 humans do not have to do anything.
There are 2 methods now....:
1. Every player goes for the full 100 points. So you have to waste some time for hosting new matches. But it is easier to count and if someone lags out you will maybe lose 10minutes.
2. Every player goes for 97-98 points. You can host less matches and save some time because you are rotating in 1 match.
Problem: If the host lags out all points are lost. You could waste about 40minutes.
I prefer the first method. Since x360a posted the second method most of the people doing it that way.
A match with 4 people takes at least 42-43 minutes. In 3 hours you can get about 400 points. For the full 2000 points (hero achievement) you will need about 15 hours.
If a player is lagging out after the match has started and it is not the host the kills for the other players will count. You just have to finish the match.
The infected is worth 3 points, a human 1 point. An assist is worth 1 point as well.