The Z4 has been discontinued. The 6900 has been the flagship of the ATN
NV scope line and is a great unit with 1/8" adjustments. The Trident is waterproof, as opposed to the 6900 which is water-resistant. Resolution is similar with all tubes. The Trident is smaller, the 6900 has a slightly larger objective lens, allowing more light. Both are overall similar performers. The trade-off is size and a bit more light on the 6900, and waterproofing and compactness on the Trident. Your choice.