I want to use an infrared remote to trigger my Canon EOS T4i for a self-portrait. Can this be done in Aperture priority (Av) mode or only in Manual mode?
You can use an infrared remote, such as the RC-6 or its generic clones, with either Aperture Priority, Manual, or any other exposure mode with your EOS Rebel T4i/650D.
The key considerations are to use the Self Timer drive mode to enable the remote and insure that the camera is either pre-focused manually to the correct subject distance or the selected AF point(s) is(are) aimed at where you'll be.
So while you don't need to necessarily use manual exposure mode, it is useful to use manual focus instead of auto focus.