GamePress
There's a level 4 raid starting in about 35 minutes.
This website lists 7 different Pokemon that can be level 4 bosses: Venusaur, Charizard, Blastoise, Rhydon, Lapras, Snorlax, & Tyranitar. Those are the only Pokemon that can be level 4 bosses? I want to prepare real quick before it hatches. :D