Its been psoted before but here it is; http://www.youtube.com/watch?v=WYvPQR49dsQ He starts off saying the advantages of it 4 at the back over 3 is that if one of the back line move forward you still have 3 at the back. Now, in the few books\articles I have read on this sort of thing, if one of the back 3 line move out, one of the midfielders drops back to cover, the Dutch even do it in their 433 when a full back moves forward, pretty rudimentary and standard stuff. Unless your stuck in a roy hodgeson rigid two lines' of 4!!