I'm gonna go ahead and say this is the start of MH4. Even though it's highly unlikely, and that these symbols in fact represent upcoming beta features in Twitter for Mac. Nonetheless, thought I'd be first to say "MH4!!!", just in case I'm right
EDIT: oh, now upon looking in the other thread about this it seems as though I'm not first. Also, I've noticed how the MacHeist logo brightens when hovering over the hint at the bottom...
EDIT 2: Just in case anyone reading this hasn't seen, here's the hint in question;
This is available not only inside Twitter for Mac, but also here, and perhaps more interestingly, here (notice the different text in the main section of the page)