I agree, 4 is sufficient.
Also, the arena community is incredibly small (currently 43 players in the season, 3 elites, rest bronze). Real-time arena games practically never happen, only turn-based arena. For me, the bluffing and intuition are relevant in real-time but get lost in turn-based. Simple games are quite easy to find with a little patience, but I'd love for more people to play real-time arena to get some good competition going. I don't really have a concrete concept in mind, but if there were some particular times when to queue for a real-time arena game that might get more people to adjust their schedule to it. There's actually a ton of people eligible for arena (I scrolled down to 2000th position and it's still Bronze-1, so should be even more Bronze-0s).
I'm thinking maybe to introduce a "happy hour" concept every 6 hours to accommodate for various time-zones. When you queue for arena a banner would appear saying either "Happy Hour!" or "Happy Hour in <countdown>"