Maybe tickets for next Mets season aren't on sale yet. I promise you, you won't have trouble getting tickets even if it was the day of, worst case scenario. Did you check if they have a home game that Friday?
I almost always get baseball tickets on Stub Hub. I'm a Yankees fan and go see their games but I can usually find tickets there around face value unless they're playing either the Mets or Red Sox.
Also why would you see a Mets game? If you're going to be in NY on April 4th do yourself a favor and see the best rivalry in baseball as the Red Sox are playing the Yankees in the Bronx. Yeah tickets will be more expensive if you don't get them direct but I think it would be worth it. After that the Yankees play the Orioles on April 12th. Both teams made the playoffs last year. Also the Mets suck? Why would you want to see them?