The lenses you have now are pretty capable of making nice images... they are just the cheapest lenses Canon makes (assuming your 50mm is the f1.8 version) and lack in build quality, AF performance and some other features. But, they are capable and usable lenses that give you a wide range of focal lengths to work with.
The 28-135 IS is a good walk around lens and a definite step up from the build quality, AF performance of your . It's an older lens, originally designed for full frame. It's just not particularly wide on a crop camera like the T4i. Still, I like it a lot on crop cameras and find it very useful. It has very good image quality (hard to distinguish the IQ from the much more expensive 24-105L), decent mid-grade build, better USM autofocus, quite close focusing and can be found quite inexpensively on the used market. A lot of 28-135s were sold as kit lenses with 40D and 50D, so it's pretty easy to find a lightly used one on Craigslist for $200 or so. Canon had refurbs of the lens at that price in their online Canon Direct Store recently, too.
In some respects the newer EF-S 18-135 IS might be more versatile single lens option, but is likely to cost a little more, even used. It also lacks USM, though the latest "STM" version likely comes close in AF performance.
The most compact is the EF-S 15-85 IS... top image quality throughout and a bit wider than most walk-around lenses. It has USM.
Any of thse are a step up from your kit lenses in terms of build quality. The USM and STM lenses are a step up in AF performance.
There are also larger aperture lenses, such as the Canon EF-S 17-55/2.8 IS, Sigma 17-50/2.8 IS and Tamron 17-50/2.8 VC and non-VC (the non-VC is the cheaper and sharper of the two Tammies). The Sigma is an HSM lens (similar to Canon's USM), the Tamrons are not and will likely be a little slower focusing and possibly a bit less accurate (Tamron is updating lenses to USD focus, but haven't done so with these lenses yet). However with all these large aperture lenses, notice that the trade-off to get an f2.8 lens is that it's range of focal lengths is a lot more restrictive, which might make it a bit less versatile. Depends upon which is more important for your uses... the larger aperture (low light shooting and shallower depth of field) or a wider range of focal lengths.
You have one faster prime (50mm) for portraiture and low light, so might not need an f2.8 zoom, too.
As mentioned, I do use the 28-135. It serves as a backup to my 24-70/2.8 and sometimes as a loaner to friends and assistants. At the prices it sells for used, I consider it one of the best deals among Canon lenses and will take the 28-135 when I'm hiking, biking, etc., saving a lot of weight by leaving both my 24-70/2.8 and 70-200/2.8 at home. I usually complement it with a Tokina 12-24/4 ultrawide, though. That makes for a pretty compact and convenient two-lens kit covering a nice range. In fact, with a crop sensor DSLR this pair makes for wider range of focal lengths in two zooms than most film shooters ever owned in their lifetimes.