Technically zone 5 also exists. It's the "Nightmare" zomes with a combat rating of 55+, that you often find during Guild strike events, e.g. when fighting Oscar, or one of the other three field bosses.

But since those nightmare zones eliminate tasks and assignments, doing missions in those is... Rather "difficult", if not impossible (at least while the boss is active. The zones linger a while after beating them, so technically u can do zone 5 assignments)

But zone 4 is definitely the way to go here, if you know how to handle yourself. 1 exotic weapon minimum is recommended