Gemini 4 didn't "walk" in space in the traditional sense. It was the first mission to conduct a spacewalk, or an extravehicular activity (EVA).

The spacewalk occurred on June 3, 1965.

This historic event was performed by Ed White, who spent 21 minutes outside the spacecraft tethered to it with an umbilical cord. He used a hand-held maneuvering unit to move around.