The Revival of 4-2-4

Recently, the two teams playing pass and move football, Barcelona and Arsenal, have switched to a 4-2-4 variation formation. And with Arsenal only 3 points behind Chelsea, and Barcelona joint top in Spain, and in the semi finals of the European Cup, there is a good chance that a major competition could be won by a team playing 4-2-4 for the first time since 1970.
What Happened to 4-2-4?
The problem with 4-2-4 is that the midfield gets overrun fairly easily. While it worked during the 60s, and fairly early 70s many managers realised that when possession was lost, it was increasingly difficult to win possession back. Because of this reason, 4-4-2, developed independently of each other in Russia and England by Viktor Maslor and Alf Ramsey. Argentina too, played a 4 man diamond. The death bell was sounding for 4-2-4, and Rinus Michels joined other managers when he realised that 4-2-4 would not work for Ajax. He played 4-3-3, and in the 1974 Final, both teams, Holland and West Germany, played 4-3-3. The age of the 4-2-4 was over, and teams were moving to 4-4-2, catenaccio or giocco al'Italia, 4-5-1, 4-3-3, or the newest formation, 3-5-2, that was developed in 1986 by Carlos Bilardo. In that final, West Germany played 5-3-2, and, as Jonathan Wilson put it, the pyramid was inverted. The days of the 4-2-4 and it's free scoring were over, as the 1990 World Cup had one of the lowest amounts of goals in it. FIFA outlawed the backpass and the tackle from behind, but the 4-2-4 still looked very dead.
4-2-3-1
Yet 4-2-4 was not dead. In fact, did it even die?
One of the last teams to successfully play 4-2-4 was Brazil 1970. However, with Jairzinho and Rivellino playing slightly behind Tostao, and Pele dropping off, what was their formation but 4-2-3-1? 4-2-3-1 is just a slight variation of 4-3-3 or 4-5-1; if it's 4-3-3, just have the wingers drop back a little, and have a midfielder advance, and it becomes 4-2-3-1. Or, it was a variation of 4-4-2; have a striker drop back, wingers push up, and you have 4-2-3-1. The credit in Spain for inventing 4-2-3-1 goes to Juan Manuel Lillo when he was in charge of 3rd Division Cultural Leonesa in 1991/92; it was transported to England when Manchester United adopted the formation after losing to Real Madrid in 2000, though Alex Ferguson claimed he has always played with split forwards. France won Euro 2000 with it, and 4-2-3-1 is now a fairly common formation; looking at the Guardian's Squad Sheets for the weekend's games, Liverpool, Manchester United, Chelsea and Manchester City are all expected to line up in that formation, and England, once the standard bearer for 4-4-2 has now switched to 4-2-3-1. And with 4 defenders, 2 midfielders and 4 attacking players, what is 4-2-3-1 but 4-2-4?
Arsenal and Barcelona
That brings us to Arsenal and Barcelona. Both teams independently came to their 4-2-4 variants after some problems with their preferred 4-3-3. For Arsenal, as described here, the issue was being suspect to counter attacks after defeats to United and Chelsea. Abou Diaby dropped back into the midfield, the wingers drop slightly deeper and Cesc Fabregas played like a ponta da lanca, off of Nikclas Bendtner (that role is now Samir Nasri's after Fabregas' injury). This article in Zonal Marking covers Barcelona's switch; here, Lionel Messi drops back, Andres Iniesta pushes up, and either Zlatan Ibrahimovic or Thierry Henry plays as the number 9, with Henry being more of a false 9. Both are fairly similar, except that with Barcelona's formation, it was a midfielder being pushed up on the left, and Messi, they're most creative player dropping into that playmaker's role, while at Arsenal, it was Fabregas playing in the playmaker's role. Both formations could easily be described as 4-2-3-1, and maybe they should be, but with the 4 attackers being higher up than they would be in a 4-2-3-1 (which can be attributed to the amount of possession both teams have), it's more accurate to describe them as 4-2-4, which, as a formation is having it's swansong before it sweeps into obsolescence for the final time.