To answer some questions I've heard from here and there:
You don't have to make a fully-fledged costume. You can make it partially.
You can make bikini ones.. sure, if they look like in game.
You can make a low-cost cosplay.
Basically we want to look at the picture and understand it's s4. So putting on a random swimwear or showing your pigtail and saying it's S4 may not be sufficient by itself.
Also, as Fera puts it... don't be inappropriate. And inappropriate isn't just showing things that facebook censors. There is nothing wrong with putting on a "sexy" pose, but if we feel the focus of the picture is the "sexyness", then we may as well DQ it. Come on, we trust y'all to show common sense ^^
Also, here is an announcement so I can't run away from it: I will be attending the event as well. Yes, there you go. I'll make my first cosplay for S4!
(and if I miss it, I'm going to make a compensation cosplay! :p)