4wd in a electric car could be "simple" to build - it dosn't even require at new or extra PEM og a "power disribution system" - the way electronic motors works in combination they will automatically distribute power to the wheels in need of power...., they (the 2 motors) will drag the power needed - the wheel with most resistance will drag the most power, if front or back is spining the other wheelset will automatically take/have more power - this is by physic lows, no need to advanced electronic handling this.
If you have a electric motor, and try stoping it turning, the motor will try work agains this change by draging more power, if the motor is spining without any resistance, it will drag less power.
So a 4 wheel drive EV will basically be a matter of adding a second motor to the front wheels and redesign the wheel base/suspension. When this is done, you need to deside 1 out of 3 options:
1. you chose to connect in serial, your 2 ev motors will share the electrical voltage evenly under normal situations by 50%/50%, both having the same amp - but if one motor for any reasoon requires more torqe it will "take" more voltage (and thereby power)
(this will ofcouse only work if the pressent PEM does not use motor/whell spin feedback in such case the PEM need to control the power distribution)
2. you chose to run them in parallel, they will both have the same voltage, but will split the amp in a 50/50 - if one motor for some reasoon reqire more torqe, it will try drain more power (amps) leaving less amps to the other - as far as I know Telsa is amp controlled, so this will be the most likely setup, of option 1 or 2
(this will ofcouse only work if the pressent PEM does not use motor/whell spin feedback in such case the PEM need to control the power distribution)
3. you add a second PEM witch give the abbility to dynamically (e.g. by the touchscreen) distribute power to front or rear in any torqe combination they like.....you chose while driving 10/90 or 90/10 with a slider like the sun roof...
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(this option is as far as I can see required if the PEM does recive motor/wheel spin feedback to control need of power, and in such case it could be difficult to control correctly - basically the 2 wheels could drive with different speeds and wear your tyres fast, and waste a lot of energy)
if option 1 or 2 is used, tesla could by design chose how tourqe spilt should be by default when all whells are force with same drag (runing on a road), simply by having different gearings front/rear - thereby insure that in normal driving the front whell will e.g. drag aprox 40% while rear wheels will drag 60% - if front or rear wheel "meets" more resistance (or less) it will automatically adjust the torqe ration..you do not need to change any thing
if option 3 is used, I will suspect they will use same gearing in front/rear and control the torqe spilt fron back/rear by software - thereby having the abbillity to let the driver dynamically select witch ration they like e.g. 20/80 or 75/25 - it will simply be a matter of software. (btw. the 2 PEM's will not nessesarlly need to be as big as the one in Models S, they could go for 2 smaller, say 600 amp each - wich will be 1200 amp together (same amp as models S sport))
if price is a matter, tesla should go for the option 2, while if they aim for the most flexible solution they should go for option 3 - sine the PEMs could be smaller, it will not nessesarlly be that mutch more expesive in HW, but will cost more to develop and test/fine tune.
all options could by drivers choise offer 100% 2WD rear or back at any time - option 3 will reduce total horse power, while option 1 or 2 could be with full horse power in 2wd since the limitation is most likely in the PEM and the cooling of the EV motors
