XLR is just a connector.
The capability of longer runs with less interference comes from usingbalanced signals. Balanced signals require three conductors and the right equipment at each end. Two conductors carry the signal, one of which has its phase inverted. A 3.5mm jack only has two conductors (mono) so one of the signals has to be dropped - therefore the benefit of a balanced signal is lost, if you simply use a converter.
However all is not lost. Beachtek and Juiced link (and no doubt others) makke pre-amps whicc mount under your camera, take a balanced signal in (from your mic) and feed out to a 3.5mm jack to feeeed into your camera. Thus the "unbalanced" signal is only a few inches (no real issue with interference) whilst the longer cable run (which is more likely to suffer interference) is balanced.
There are several benefits to the H4n, some of which:
Great for picing up a good stereo ambient sound track, using inbuilt mics (also not bad for "table top" interviews.
Use with a lavalier mic and put in the pocket of a subject whilst shooting from a great distance (avoiding wireless connections - but be careful as you cannot monitor)
Ditto above but hand held mic for roving reporter
Sometimes it's just useful to have a backup audio.