I'm going to start by saying 4231 is, and has been my favourite formation since FIFA 15 Coins.
As I do at the start of every FIFA, I make a cheap team in 4231, then gradually build on it throughout the year, trying to attain top players, trying out players that look desirable in the process in order to find the perfect team. Luckily, I packed Hazard and Neuer this year, and was able to try out a lot of the best players for this formation very early into the game.
Why use 4231?
If your main game is based on counter attack, fast paced football, this is not the formation for you. This formation is great for ping pong passing and keeping possession. The main difference between 4231 and 4231(2) is that 4231(2) is more of a counter attack formation with the wingers (usually faster than wide CAM's) out wide ready to break as soon as you win possession back. When you win possession back with the 4231, the central CAM and the STR are going to be the only 2 up front that will be ready to break for a counter attack, as the wide CAM's will usually get caught by the opposing fullbacks and are usually checking in to receive the ball.
Your fullbacks will get involved in your attacking play quite a bit in this formation, because even if you were to lose possession with your fullback in the opponents third of the pitch, your CDM on that side will provide cover. Try to get a fullback with H/M workrates and decent dribbling. Defence wise, just make sure he is good defensively.
Recommended players: Marcelo, Alba, Coentrao, Baines, Dani Alves, Juanfran, Piszeck
WORKRATES MATTER. Last year, my favourite CB partnership was Pique and Hummels. They were both H/M, but I found that to be great. This year, it is suicide. Having one H/M CB is fine, but make sure the other is L/H, M/H or at least M/M. Also, defending this year is quite a bit different from last year, so don't just look for pacey CB's, look for CB's that can actually defend. It will benefit you in the long run. But hey, if they are pacey and can defend, double trouble!
Recommended players: Hummels, Ramos, Pepe, Chiellini, Barzagli, Kompany, Pique, Silva
This is where the formation gets interesting. Having 2 CDM's that are super defensive give maxes out your teams defense, but takes away from your teams offense. Having 2 CDM's that are super attacking on the other hand will max out your teams offense, but leave a huge gap between the midfield and the defense. It's important to find a perfect balance in workrate in your 2 CDM's because they are the unsung heroes of this formation. They are the key players you pass back to from the front 3 CAM's so make sure they have good ball control. That is a vital point. You may or may not notice them throughout the game, but they will make vital passes, interceptions, assists and even pop in some goals here and there. They are the heart of your team.
Some good setups would be:
M/H - H/M
M/H - H/H
M/M - H/M
H/H - H/H
Recommended players: De Rossi, Fabregas, Schweinsteiger, Lahm, Alonso, Busquets, Toure, Ramires, Vidal, Matic
These are your "wingers". Many times I come up against 4231 teams with converted, fast, strikers as outside CAM's. Though this does spice up the attack, it greatly damages the possession style and turns it completely into a counter attack formation. If that is what you wish to do, feel free, but I do not recommend. They will get a few chances on goal, so make sure they can finish. Dribbling and Acceleration are the key stats in this position because switching direction and cutting in quickly is when attacks become vital, as you draw CB's toward you, which triggers runs from the other CAM's and striker. The only workrates I would stray away from for this position is H/H and M/H as you don't want your wide CAM's playing RB/LB on counter attacks. I would also recommend 4* skills, but it isn't necessary.
Recommended players: Buy FIFA 15 Coins Mata, Gotze, Ozil, James Rodriguez, Isco, Sneijder, Mertens, Kagawa, Messi, Iniesta, Hernanes, Diego
*When I say CF below, I mean a second striker as CAM.*
There is a lot of versatility to this position in particular depending on whether you want him to act like a 3rd CAM or a CF. Personally, I use him as a 3rd CAM, as it makes it easier to keep possession. Having him play as a CF however, will make this a deadly formation on the attack as you have 2 goal threats instead of just 1. Having said that, goals in general are pretty equally spread out amongst the team, but having a CF here would have him see more goals than if he were a 3rd CAM. If you choose to use a CF, make sure he has good short passing.
Recommended CF's: Benzema, Jovetic, Suarez (I know hes not out yet but he looks immense for this role), Tevez,
Recommended CAM's: Fabregas, Rakitic, Kroos, & everyone mentioned in the Wide CAM's section
To be honest, pretty much any striker can adapt to this role because he usually gets fed through balls and just has to finish. Usually doesn't have to make chances for himself, but if you have a striker that is capable of doing so then go right ahead. Finding the perfect striker to fit your playstyle however is what makes this a hard position to choose when team building.