The T4i's noise profile is pretty good, though like many cameras it fares a lot better in focused areas than out-of-focus ones. Here, it preserves clean detail until about ISO 400, at which point it starts to degrade.
Color settings differences aside -- and this is one case where I like the deeper tones produced by the default
Auto Picture Style -- you can do better processing the raws at ISO 800 (bottom) than with the JPEGs (top).
(1/80 sec, f1.4, ISO 800, evaluative metering, AWB, 24mm f1.4 lens)
As with most cameras, in-focus areas at high ISO sensitivities look pretty good, but out-of-focus areas display lots of artifacts, including hot pixels.
(1/50 sec, f2.2, ISO 1600, evaluative metering, AWB, 24mm f1.4 lens)
The T4i fares worse than the T3i at higher ISO sensitivities, either because of lower sensor response or in-camera overprocessing of the blue channel.
(1/30 sec, f3.5, ISO 1600, spot metering, AWB, T4i: 18-135mm lens at 19mm; T3i 18-55mm lens at 18mm )
Although you can see the noise when you pixel peep, even at ISO 3200 the images are usable if there aren't a lot of shadow areas in them.
(1/60 sec, f2.2, ISO 3200, evaluative metering, AWB, 50mm f1.2 lens)
The default Auto Picture Style really pushes the contrast and saturation to the point where hues shift slightly and you start to lose shadow detail.
(1/100 sec, f6.3, ISO 100, evaluative metering, AWB, 18-135mm lens at 35mm)