I don't hear sound when playing WAV or MP3 file on PC when using input 1.
Mode: STEREO
Input: 1 and 2
XLR connected to Input 1
Format: WAV 44.1Khz/24bit

I can hear sound from input 2 right channel.
Is my H4n a defective device?
TIA.

So should we assume that your recorder works properly (two separate stereo channels) when recording with the built-in mics? Does it work when you play the recording on the H4 itself (vs. downloading and playing on your computer)? Does it work when recording 2-channel stereo through the XLR inputs? Did it every work? How old is it? Not clear whether you have done any of the common differential diagnosis troubleshooting steps?

From what I surmise, (???) you would need an XLR 'Y' cable to feed both L&R inputs to get sound to both channels, if it does not have the dual mono recording mode like the H2. Though there is probably a work around right in the H4.
You can always convert the file with your favorite audio software.

If you only connect one XLR-channel then you have the option 'mono mix' which duplicates the sound to the other channel. This should work in Stereo and 4CH mode. Press 'menu', select 'input' and change 'mono mix' to 'ON'. Hope this works for you.

are you feeding into the xlr/1/4 plugs at the bottom? Like Stany said use Mono-mix to record to both channels.