Specifications for a charger needed to charge LiFePo4
4200mAh is 4.2Ah, or 4.2A for 1 hour. Therefore to charge the pack in 1 hour requires a charging current of 4.2A. To charge 3 packs in the same time you either need 3 chargers that each do 4.2A, or a single charger that does 12.6A (only if all 3 packs will be permanently wired in parallel).  

4S means 4 cells in series, which at 3.3V/cell gives a pack voltage of 13.2V. However at full charge the voltage may go up to 3.6V/cell, so the charger should put out a maximum of 14.4V. 

When a LiFePo4 cell reaches full charge the voltage rises rapidly. If all cells in the pack don't reach full charge at the same time and the charger only looks at the total pack voltage then some cells may not get a full charge. Therefore the charger should monitor the voltage on each cell and equalize or 'balance' them until they all get to 3.6V.    

If you are looking at purchasing a ready-made charger then choose a model which does 4S LiFePo4 and has a balancer. If you want to make your own then at minimum you need a power supply that puts out 14.4V with adjustable current limiting, and a balancer board for each cell. 

The balancer board is basically just an amplified Zener diode that limits the voltage to 3.6V. Many have an LED to show when the cell has reached full voltage, so when all 4 LEDs are lit you know the battery is fully charged.