With an APS-C camera like your T4i, 50mm is a good focal length for a portrait lens. This gives you several decent options:

• Canon EF 50mm f/1.8 II for only around $115. This is a cheap, plastic-body lens, but it benefits from the ease of making lenses in this focal length, and years of history. This is actually my top recommendation, as it leaves you room to buy a flash as well, and that will probably make the most difference.
• Canon EF 50mm f/1.4 for around $340. This has a better AF system and, of course, a slightly faster aperture.
• Sigma 50mm f/1.4 for around $450. More money, but a brand new premium design. I haven't actually used this one but it's getting good comments.

There's a nice review of the 50mm f/1.4 at DPreview, which touches on all three of these lenses.

The Canon 40mm f/2.8 STM is another interesting option. It's also cheap at $150, and optimized for video. The wider focal length makes it more useful as a general-purpose lens and it's still a decent portrait range. If you're looking for a choice with a lot of versatility for things other than portraits, this would be a good one.

You may also consider the Canon 85mm f/1.8 EF USM, at about $420. This is still in a decent portrait range; a little tighter, obviously. While it's not quite like its $2000 f/1.2 big brother, this is a classic and highly-regarded lens.

But, as I hinted above, getting a new prime lens isn't necessarily the first thing to do. You could just keep your existing lens, which covers the portrait range nicely, and add a Speedlite 320EX ($225) or Speedlite 580EX II ($490). This won't necessarily get you the fashionable thin depth of field look you might get from a faster prime, but will let you take more control over the light in your portraits. I'm not really one to discourage buying primes, though, so the 50mm f/1.8 plus Speedlite 320EX makes a very attractive in-budget combo recommendation.