In Chapter 4 of The Great Gatsby, what "matter" did Gatsby have Jordan Baker discuss with Nick?
When Gatsby learns that Nick, his next-door neighbor, is Daisy's cousin, and when he realizes that Jordan knows Nick, Gatsby prevails on Jordan to ask Nick to arrange a lunch at his (Nick's) house. This lunch will be attended by Gatsby, Nick and Daisy, who Gatsby hasn't seen in five years. Gatsby wants Nick to keep Daisy in the dark about Gatsby coming to the luncheon: he wants to surprise her. When Nick asks why Gatsby doesn't simply have Jordan arrange the lunch and leave him out of it, Jordan responds that Gatsby wants to show Daisy his house. That wouldn't be easily possible if they met in the city, but from Nick's house it is an easy walk to Gatsby's mansion.
From the conversation, it is clear that Gatsby has thought this through very carefully. It's also the first time Nick learns that Gatsby bought his house out of love for Daisy, and, thus, the first time Gatsby's story starts to make sense to him. Also, we should note this is a highly mediated conversation: it is Nick's account to us of what Jordan told him, so we are more than one layer removed from actually witnessing what Gatsby—or Jordan—really said.
Gatsby is determined to get Daisy back and in order to do this he has to meet her again after five years have gone by. Gatsby is aware that Nick Carroway lives next door to him in West Egg and that he is cousins with Daisy Buchannan. When Gatsby speaks to Jordan privately at his party, he asks her to ask Nick if he can set up a meeting between the two of them – Gatsby and Daisy – so that he can meet her again and try to begin a relationship with her.