If more are discovered, I'll update this page. Helpfully, if you're using a controller, it pulses as you're approaching a media stick.
You do get a t-shirt and an extra mix if you collect all four, at least: the CLR Launch Party mix. So far it looks like there are only four of this new type of collectable to find, which is certainly less than Rockstar's other fiendish scavenger hunts, like the broken radio antennas or movie props.