I have just purchased a Zoom h4n and I am very pleased with the performance. I am using it underground in Caves and Mines. I also use Sennheiser ew 100 G2 Radio Microphones. They work perfectly ok into the Ext Mic socket on the rear but I am not able to get any input via the Input 1 /2 in the 4CH Mode. As I would like to be able to record both sets of Radio Microphones. Is it possible.
There's some weird bug with the phantom power on the H4n. I'm almost certain that's the issue you're having. Try switch off the phantom power.
Did you press the buttons 1 and 2 on the side (they should be red when illuminated) and bring the gain up after selecting those inputs?
Bare in mind that when you plug into the quarter inch jack in the rear, you disable the onboard mics.
Hope this helps.