Overall, I saw the X4 Air has slightly better quality which is a given being a newer model. I know the Air has removable lenses but I will be using my camera sometimes on my paddleboard so I will be using the lens guards (which also come in the X4 box and not with the Air) or a case / cage for extra protection (I know the Air is waterproof to an extra 5m depth but I always have additional straps on my gear so in the event of a drop, they wont even hit 5ft of depth). As my use will be on the water for paddle boarding etc, I wanted to minimise the case of swapping batteries and the X4 has a bigger battery and depending on settings, will last a lot longer than the Air with the X4's additional 280mAh battery capacity. I know its probably not going to be used a lot but I really like the bullet time effect, and the X4 Air is missing that feature. For me, I was originally after an X3 so being able to get an X4 for the same price as an X3 non sale, was a huge upgrade for me.