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"The idea is that the arms are positioned from an engineering standpoint to transfer power as best they can – taking the positions in the pedal stroke where we tend to apply most power into account – while allowing Shimano to reduce their number of arms from five to four, and thereby save weight.
Shimano reckon this is as stiff as previously, but lighter (the 7900 version, including bottom bracket, was 725g; this version, including bottom bracket, is 683g). Did I notice any difference in the rigidity when riding? Nope, certainly not.
The bolt circle diameter is 110mm whether you go for a standard (something like a 53-39T combination) or a compact (50-34T) – which makes things easier. You could ride standard chainrings at home, for example, and swap to smaller chainrings for a week in The Alps, for example, using the same cranks."
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hippy
The difference is how much material is required to do the job. Ye olde five-arm spiders had a nice aesthetic, but they weren't optimised.
The 3D chainrings pioneered on the last 10s groups paved the way for ditching the 130BCD; unfortunately this is getting a bit fancy for a consumable part though. If you look at aftermarket four-arm rings, they use traditional construction with big shaped nuts both for styling and to reinforce the ring against shifting force; that seems like a good alternative to overly fancy 3D rings.
But when you look at the placement of the four arms in relation to where the rider's torque is delivered through the revolution, and then you notice that one pair of arms is wider than the other, corresponding to the top half of the power stroke, it's apparent there's no more improvement to be made - it's function over form.
And if you think about it, if you include a consideration of the torque profile coming through the cranks, this design has more symmetry than five arms.
On the other hand, the styling over the top of that form is certainly a bit wank, and arbitrary slight differences in the ring lands between tiers preventing mix and match is just bullshit... Hang on - the 9200 ring you posted - I just noticed they've gone to the same width for each arm!
Now that seems like change just for the sake of it.