Here's the announcement:
L4 is intended for the mid-size in this announcement.
Current buyers of Air and Gravity paid for what the specs say. The current specs do not mention L3 or L4 at all. Someone may have speculated that they paid for access to Air and Gravity L3 and L4 but it's all verbal and not a contract of what we paid for.
My speculation is: L4 requires more powerful hardware and software that the Air and Gravity are not designed for (thus, mid-size). However, the hardware for Air and Gravity is good enough to achieve L3.
My Samsung watch 7 can't measure Antioxidants. I have to throw it away and buy version 8 or above to get that feature.
In summary, we paid for the current specs and none of us is cheated just because the future hardware/software keeps changing.