can i record with a zoom 4n simultaneously with the 3.5 mm jack and the built in mics and the xlr jacks at the same time?
Definitely not! If you insert a plug into the 3,5mm jack the internal mics are bypassed. But you absolutly CAN record 4 channels at a time, even using 4 external mics. No phantom power on the mini-jack-input obviuosly, but the Rode NT4 doesn't need any, for example. Or you use external phantom-power supply boxes.
Interesting detail on the Zoom H4n's mini-jack input: The pre-amps on this input have a much lower selfnoise than the more professional-looking XLR-inputs. Seems kind of counter-intuitive at first, but it seems they thought that most people would use the onboard mics most of the time... then it starts to make sense.