Just my opinion, but numbers 2 and 4 shout "look over here!", which makes me want to look at the avatar rather than the post content. Of the other two, I would say #3 looks the best. Ultimately, though, the choice is yours
#3, looks like the Apple Music icon. Now, time to change all the avatars back to square, with the exception of a nice iOS 7-style rounded rect for your own