Ivan Angeli just wrote me to say he's seen Series 4 Collectible Minifigures on sale in Belgrade, Serbia at the store 'Hipopotamus' near the Tashamjadan Sports Center!
This is the first official sighting that I'm aware of. It would stand to reason that Europe will receive this round first followed by the rest of the world. We'll just have to see if those of us in the US will need to wait until April 15th, or if a North American launch is eminent.
Big thanks to Ivan! I've added the first pin to the Series 4 Rollout Map. Remember that you can add 'virtual pins' too when you find Series 4 in the wild. You can see how to do so here.