FYI, since you seem to be new to Lipos,
Most LiPo/Lion cells are 3.7V nominal (4.2V max and 3.2V min), so a 4S pack should go up to 4*4.2V = 16.8V at full charge and go down to no less than 4*3.2V = 12.8V at full discharge.Oct 14, 2012
Is this a good choice to power the 2 x 12V motors + all other components ? cause I'm worried.. about 14.8v if it would damage the motors.
NO.
Use 3S (12.54 V at full charge, can be as low as 3*3.2V = 10.7V).
Not a good idea to overvolt the motors. If you were using some kind of closed loop feedback you could regulate the voltage using the PWM duty cycle. I haven't seen any vendors link for you motor shield so I can't comment on that. In fact , I haven't seen anything from you on how you actually plan to control the motors. No mosfet part numbers , no code, nothing. It is way to early to talk about controlling the voltage of a system that hasn't even been designed yet. As far as I can tell, you have the cart before the horse. Where I work, they design the system, have engineering design reviews,
finalize the design, then order parts. I don't know where you learned electronics but they must do things backwards there. Sorry if that doesn't sound too complimentary but I call it as I see it.
Post a schematic of your system and then we'll talk. Here on the forum , we do things the professional way. You may be a hobbyist, but we are electronics professionals, not hobbyists. We start with a feasability study or a preliminary design and refine it from there. The schematic is introduced early on, if only in a block diagram and then a real schematic. The code should start with an algorithm, and then progress to actual code. We don't just throw stuff together and turn on the power. That's what hobbyists do, which explains why they wind up here asking for help.