2 megabit cart, do you mean? Ganbare Goemon, released in July 1986. I think. People said Makaimura was the first 1 megabit game when released in June 1986, but reportedly Gumshoe was released in the US the same month (that was a 1.25 megabit ot 160KB game, I believe)
Odd then that the first 4 megabit Famicom game seems to be the first Ganbare Goemon Gaiden, released as the first game of 1990.