Karnak, the P-51s, the 109s, a noticable number of craft were updated to a second, intermediate, level of advancement.
It is enough to say they are night and day to their 256x256 "1st gen" predecessors, and again it was night and day from them to the newer "3rd gen"...
Although now we are in a fuzzy state, IMO. I'd say we're at least 3.5 gen of bitmap graphics. We have refined bitmap layouts on new craft, more control over how they look, and in some planes even higher resolutions! (B-29 has 4x the resolution because it's so big, at 2048x2048). We get things like more detail in the damage model (gear doors, without losing the gear... Shredded flaps without the flap being gone, etc).
However it's still very closely tied to the changes we saw with the B-24/T-34/Ki-84, so I consider them "3.5" instead of flat-out "4".
My thoughts on the matter...