Who was Sue cosplaying as in Genshiken Nidaime episode 4?
This was when she cosplayed when they were trying to get the doujin finished and she wanted Ogiue to cosplay as Azusa.
In this episode Sue is cosplaying as Comic Master J, the titular character in the manga of the same name. Here's a comparison (based on manga, but clearly the same character):
Comic Master J is essentially unheard-of in the west, but it had 13 volumes and was reasonably popular and well-received in Japan. There's no Wikipedia page in English, but the Japanese article has some nice details. Here's a brief summary (written by me, adapted from several sources):
Comic Master J is a mysterious super assistant-mangaka who can instantly finish the production of any manga for 5 million yen, so long as it is made with "soul". The story follows Comic Master J and the various mangaka he helps, gradually revealing more about him and the various mysteries that surround him