Specs on GSMArena:
Samsung I9500 Galaxy S4 vs. LG Nexus 4 E960 - GSMArena.com
GS4 has a bigger screen with a higher pixel density (not necessarily BETTER, depending on how much you like AMOLED+PenTile), the smallest memory capacity of the GS4 is as big as the largest memory capacity on the N4 and it can take an SD card, removable battery, better camera, next generation of Gorilla Glass, more sensors (Temp, Humidity, Gesture), MHL2 TV-Out, Infrared out, LTE. If you want to compare it strictly on hardware specs, honestly, it's not really a contest.
Unlisted advantage of N4 is pretty obvious - it's a NEXUS. 4.2.2. Photosphere. Vanilla-y goodness. 'nuff said. At the moment, the GS4 is also running 4.2.2, but Nexus will go to the next release long before the GS4. And Google has still subsidized the hell out of it, so if you don't want carrier commitment you'll never EVER get as much phone for the money.
If you laid both phones in front of anyone and offered them up for free, I think most people would be best served by grabbing the GS4, unless you are a developer or always want the latest Android no matter what.
If you have someone who is OK with a 2-year carrier lock-in, I think the GS4 is probably a better choice for less money than an N4 (with the same exception as above).
But if you have someone choosing between the two phones off-contract, they'll have to justify three hundred extra clams for the features listed above. Is it worth it? Only the person potentially spending the 300 extra clams can answer that. Personally, for me, I think it would be. But I likes me a removable battery, I occasionally find myself with my phone as my only camera and I want to capture the moment in the highest quality possible, and $300 isn't a life-changing amount of money in my budget.