A DIY geotagger is feasible, but not as a direct GPS accessory to the camera in any standard way. Based on the answers, Canon bodies like the T4i don’t offer a simple, standard interface for an Arduino GPS unit to inject GPS metadata directly into each image.

The practical DIY approach is to log GPS positions separately and sync them later using the photos’ timestamps. One option is simply using a phone app to record a GPS track, then matching that track to image EXIF times on a computer.

If you want your Arduino device to know exactly when a photo was taken, the hot shoe can be used as a trigger: on a standard ISO hot shoe, the center contact and shoe ground act like a switch that closes when the shutter fires. An Arduino can detect that closure and record the current GPS position at capture time. You should include basic protection circuitry so you don’t risk feeding unsafe voltage into the camera.

So: yes, you can build a useful DIY geotagging system, but the realistic method is logging GPS data externally and writing coordinates into the files later, not sending GPS data directly into the camera body.