Welcome to my 4312 guide. I've been using the 4312 for about the last 4 months, and had something like a 4:1 WL ratio with it, winning div 1 the majority of the time.
The trick to being succesful with the 4312 is to make it play like a 41212, and we do this via your players work rates.
The whole of the defense is best off as being M/M. This means that you'll always have your 4 defenders back defending, and your full backs will also occasionally help going down the wing adding width to the team. Players like this are De Ceglie, Gibbs, Caceres, Sagna, Riether, Fanni, Jose Enrique and Konko.
Around 80 pace is fine for the full backs, and for your CBs you want something around 74+ pace, and to be good in the air like most CBs.
Most important part of the team to get right. You want your wide CMs to act like wingers, so you want them to have a good deal of pace, and to have H/L, or H/M workrates. Players like Menez, Nasri, Sneijder, Wijnaldum, and generally any player with around 80+ pace, and a high attacking workrate are perfect.
The middle CM is your defensive midfielder, so he needs to have a higher defensive than attacking workrate. Pace is NOT an issue here. I've used Xabi Alonso with much success. Players like Pogba, Gonalons, Xabi Alonso are all good. You want them to be reaosnably tall, strong and have a good longshot.
The CF MUST have a cracking shot on them, and a H/M or M/M workrate is preferable. Players like Wijnaldum, Benzema, TOTS Cabella are all good choices. Pace isnt that important here, but 75+ is fine.
The strikers, it's vital that both of them have a high attacking workrate, H/M preferable. As usual with strikers, a bit of pace is vital.
Some examples of teams which I use:
Cheers for reading, and enjoy!