you are not to be concerned about the rails or amps.
the 4 pin conector should not consume more than 5 amps at max.if it was to exede that number you could kiss your pc goodbuy.
but lets say yes it needed all the 12 volt are soldered together in the power supply which means you whould be able to get the 39 amps from one conector.this will never happen but in theory it is possible.
as for the gpu if a gpu requires 250 watt what whould have happened then ? well pure physics Kirchhoff's current law (KCL) if you know basics in electronics this should befamiliar to you
lets say i4 is not existent and no resistors are on the diagram.
lets say i2 5 apms and i3 5 apms are your 12 volt rails thye both conect together you whould have 10 amps on i1 or power conector.
if i4 was not in use like the 4 pin conector then there whould be no current going through it so both of them whould have went to i1 again.