Many coaches love this style of defending, but there are some very important considerations that must be taken into account before its implementation.
The flat back four is a great way to reduce the playing area/space of the attacking team, as it makes the field shorter. The system does not make the field any narrower, so opponents that are very skillful and athletic on the flanks must be respected. The reason being you are now playing without cover.
You must also have the right players. Your outside backs must be very good 1v1 defenders. They must also have very good recovery speed to protect against early diagonal switches from the opponent.
Your central defenders also need pace and must be very good communicators and readers of the game. They must recognize when to step and when to drop off.
The goal keeper must be very good with his feet, as he/she must stay connected to the back four. He/she must be comfortable playing higher off the goal line in order to neutralize balls played over the top.
The biggest mistake novice coaches make is to teach this system without teaching the principles of defending first. They rely on the system to defend for them and the players remain naive technically and tactically. Some coaches are also blind to the prerequisites that make the system successful.
This leaves us with the big question. When do we teach it, if I have the players to play it? My advice would be, when the players have a thorough understanding of all defending principles. I think U14 is a good age to start.
For more information about the best ways to introduce the flat back four, check out this video.