All the Rebel/xxxD/xxxxD series cameras use a "penta-mirror" instead of a true pentaprism. This is to reduce cost, size and weight.
All the xxD and xD models use a true pentaprism, which makes them a bit brighter.
7D-series and 70D use a transmissive LCD focus screen that partially offsets the brighter VF (darkens noticeably when not powered on).
Your T4i has a 95%, 0.85X magnification viewfinder.
70D's is 98%, 0.95X. And, for comparison, 7D/7DII's are 100%, 1.0X (probably one of the main reasons the 7D-series are about the same size and weight as a full frame 5D-series camera).
Your T4i has 9-point AF system with all nine cross type (center point enhanced), f5.6. It has All Points/Auto Selection and Single Point/Manual Selection AF patterns.
70D has 19-point AF system with all cross type (center point enhanced), f5.6. It has All Points, Single Point, and Zone AF patterns.
7D has 19-point AF, all cross type (center point enhanced), f5.6. It has All Points, Single Point, Zone, Expansion Points, and Spot Focus patterns.
7D II has 65-point AF, all cross type (center point enhanced), f8, with All Points, Single Point, Small Zone, Large Zone, 4-Point Expansion, 8-Point Expansion, and Spot Focus patterns.
T4i doesn't have Micro Focus Adjust. 7D has 20-lens MFA. 70D and 7DII have 40-lens MFA. Latter two cameras also have dual adjustment with zooms (20-lens version is only one adjustment per lens, regardless).
There are various AF performance tweaks that are user adjustable with 70D and 7D-series cameras. A lot of these pertain to AI Servo focus for action/sports.
Your T4i uses a Digic 5 processor. 70D uses a Digic 5+. For comparison, to support high frame rates 7D uses dual Digic 4 and 7D II uses dual Digic 6. The 7D-series also use a discrete chip to run their AF systems (same as 1D-series cameras). Your T4i and the 70D use a single processor for both AF and to handle images.
70D can shoot up to 7 frames per second. 7D, up to 8 fps. 7DII, up to 10 fps. Your T4i can shoot at 5 fps.
Your T4i is rated for up to 30 JPGs or 6 RAW files in a burst (after which, the camera has to pause to clear the buffer). 70D is rated for 65 JPGs, 16 RAWs. For comparison, 7D is rated to 130 JPGs, 25 RAWs (firmware 2.x) and 7DII 1000+ JPGs, 31 RAWs. All these ratings are the respective camera's peak performance, based on the fastest capacity memory cards the camera can fully utilize.
70D has articulated LCD screen, same as your T4i. 7D/7DII do not.
T4i shutter has 1/4000 top speed, 1/200 flash sync. 70D and 7D-series have 1/8000 top, 1/250 sync.
Canon doesn't publicize shutter durability ratings for all cameras, but basically it's safe to assume Rebel series are 75,000, xxD series are typically 100,000. 7D is 150,000 and 7DII is 250,000.
All in all, there are quite a few differences to consider, even if image quality and high ISO performance aren't all that different.
EDIT: Regarding flash... I don't know why people are afraid to use it. After a pop or two, kids and adults tend to ignore it. Most animals completely ignore it, so long as you aren't too close. IN fact the noise of the flash (the "pop" as it fires and whine as it recycles) often seems to bother animals more than the brief, bright flash of light.
Watch an experienced event shooter at work. They typically don't use bounced flash. They mostly use direct flash, sometimes with some sort of diffuser or modifier.
Bounced flash introduces too many variables and seriously reduces the power of the flash. Often the bounce surface isn't an ideal color and will add ugly color casts to images. When bounced, the light has to travel a lot farther and some of it is absorbed by the bounce surface, wasting a lot of the flash's power, forcing the flash fire more fully and in turn causing slower flash recycling, more rapid battery drain.
It is much more controllable and reliable using direct flash.
But the flash built into many DSLRs pretty much sucks. It's underpowered, drains the camera's battery rapidly... and being close above the lens it's in the worst possible place for redeye and ugly shadow effects.
Much better is an accessory flash. Even the smallest are more powerful than the built-in and they have their own separate power source(s). Put it on flash bracket off to the side and higher up, attached to the camera by an off-camera shoe cord, and that will help eliminate a lot of redeye and shadow problems. But in most cases because it's taller than the built-in, an accessory flash mounted even directly in the hot shoe is an improvement over the built-in flash.
Canon ETTL/ETTL II flash control can be very simple and easy to use, too. Just remember that in any of the camera's auto exposure modes (Tv, Av, P) you'll get FILL FLASH. In these exposure modes, the camera still exposes for ambient light, and the flash is fired at a reduced output, about -1.33 or -1.66 stops.
To use FULL FLASH, set the camera to M. So long as the flash is set to ETTL or ETTL II, you actually still get auto exposure, but with the camera set to M it will be based upon the flash alone, the camera will largely ignore ambient light. You can change ISO, shutter speed and aperture to increase or decrease how much ambient light is recorded along with the flash.
Many Canon flashes also can be used manually... And most have + or - Flash Exposure Compensation (FEC) that works a lot like Exposure Compensation (EC) on the camera itself. This allows you to fine tune the flash exposure. Many Canon flash/camera combos also offer Flash Exposure Lock (FEL), which is similar to using AE Lock (AEL) on the camera itself.
When using FULL flash, it essentially "becomes the shutter". You can use any shutter speed at or slower than the flash sync speed (you just have to watch out for how much ambient light it will allow to be recorded in the image). The flash itself fires at the equivalent of about 1/720, so will act to freeze action in most cases. The ISO and aperture control the distance the flash is able to reach (and many Canon flashes show you the flash range at the settings you've selected, right on an LCD screen on the back of the flash... if not sure the distance, focusing your lens on the subject can act as a "rangefinder" of sorts, so long as the lens has a distance/depth of field scale).
When using FILL flash, you do have to be a little cautious with moving subjects. Too slow a shutter speed can produce "ghosts" because you are recording both ambient and flash light together. This can sometimes be used to good effect, though, to show movement... But you might want to use Second Curtain Sync, or it can look odd.
Camera flash sync ratings aren't entirely the limit of possible shutter speeds, either. Many Canon flashes and flashes for Canon provide High Speed Sync (HSS), where the flash adjusts itself to work with shutter speeds above the camera's rated flash sync speed. This does reduce the distance the flash can reach, though... A lot at higher shutter speeds. HSS and 2nd Curtain Sync can't be used at the same time, either. (But there is little reason to use 2nd Curtain Sync with faster shutter speeds, anyway... It's mostly needed with slower shutter speeds.)
I highly recommend experimenting with flash... Especially with a reasonably good and more powerful accessory flash, ideally mounted on a flash bracket and attached with an off-camera shoe cord, so that it can preferably be used directly, not bounced.