Smaller and lighter.
The camera makers were thinking of you when they developed the mirrorless DSLR camera.
Will they outperform the Canon T4i ... No.
But they can perform as well as the T4i.
The difference is they cannot duplicate the advantages of the through-the-lens (TTL) optical view.
That being the very clear image through the viewfinder that makes it easy to do a manual focus, the ability to see the view with a brighter aperature setting than the setting the camera will use for that picture and generally the better low light performance of the viewfinder.
Your next question is likely to be "which mirrorless camera performs the best".
Very few people can answer that question accurately because very few people have used all of those cameras.
The most knowledgeable people are the people who do digital camera reviews because they have used more cameras. So you should read all of the reviews you can find and decide which is better for you.
Video is generally pretty good on DSLR cameras, but audio is generally poor.
Video/Audio is treated as an after thought and they don't want to make the audio better because it will increase the cost of the camera.
There will soon be a very few amateur cameras on the market that will combine the good features of the DSLR camera and a Camcorder. But the price will be well over $2,000, the more professional models will run over $10,000.
If you want a camera that offers good picture quality,features,good video but without the size of dslr,is any of the mirrorless that outperform the t4i