The 4" dome is an interesting point. This might be housing dependent as upon a quick look I couldn't find a Zen 4" port for Aquatica. They may exist, and I'd be greatful if someone can point one out. I'm interesting in small dome for tec dives as it's more manageable. The Aquatica 6" doesn't seem small enough to justify buying it when I have an 8"
Aquatica makes a 4" dome similar to the Zen at about $200 cheaper.