I consult for reviews on professional photography equipment so maybe I can shed some light on the differences with some technical info.
While both cameras record in a resolution that is 1920x1080, that is completely meaningless.
You will not get 1920x1080 resolution in quality video without spending at least $7,000 +lens. Period.
All produmer/consumer 1080p cameras either do not record true 1080p or record the actual resolution with noticeably terrible quality in other areas (color,moire,calibration, etc).
The iphone does not even record 1080p video. It's closer to 720p, due to lens constraints which is fine because the T4i records what most people call 720p and then upscales it to 1080p. So in the end they are both 720p (keep in mind good 720p is better than bad 1080p and many tv shows are shot with DSLRs that shoot upscaled 720p )
Anyways the first major advantage of a dslr vs an iphone is the sensor size. Large sensor size means less noise, in fact at identical apertures the iPhone 5 has 800 (eight hundred) times more noise than a T4i. Of course you don't often shoot at those apertures so practically speaking you'll have around 40 times more noise which is still a lot.
The other thing a large sensor does is enables a shallow depth of field.
Next there is the lens of the iphone. Besides having very poor resolution (but still very very good for its class) it has tons of bloom and very low contrast. What this means in practice is that you will have a grayish washed out video with an iPhone and you can't just up the contrast as the contrast is unevenly distributed through light bleed and hazing (though high end DSLRlenses aren't immune to this they have it under control thousands of times better).
Next you have the video compression which washes out the detail of iPhone videos.
Also don't think that noise and resolution aren't related and that you're getting a 720p video camera still compared to a DSLR. Every time you halve the amount of light going into the camera after base ISO you will also reduce the linear resolution by around 3% due to the fundamental way which the physics of light and image noise work (when you have more noise you can make out fewer details). What this means is that when the T4i is shooting 720p at base ISO. You will only have 54% of that resolution on the iPhone at the same aperture. Which should happen every time it's fully overcast or darker with the iPhone.
On top of that you cannot mechanically zoom (digital zoom is actually just cropping so if you zoom in x2 you get a quarter of the resolution, the iphone has a fixed lens).
The iPhone 5 doesn't have issues with color accuracy though which is nice.
So as long as you plan to shoot video exclusively at one focal length and aperture (iPhone has a fixed aperture) on bright sunny days and don't mind terrible bloomy lack of contrast and lots of video compression the iPhone will be as good as the T4i. In the 99.99% of situations that don't meet those standards it will be much worse.
Hope that clears things up.