This seems to be happening on a bi-monthly basis now, with the 4 Shadow Microns and 4 Red Microns each being incentive bonuses at select Japanese stores.
At the end of this month (Sept 29th), another 4 Microns, at 4 different JP stores/chains, will be released.
The gimmick this time isn't 4 different moulds in one colour, like previous campaigns... but 4 different colours of the same mould (Ratchet's Micron), which can join together to form a shield.

It's called the Rainbow Shield, as the four colours are Purple, Red, Yellow, Green.

Red R.A. - spend 3000 yen on Arms Micron at Toys R Us.
Green R.A. - purchase Arms Master Optimus at (Toys ion).
Purple R.A. - spend 3000 yen on Arms Micron at Bic Camera.
Yellow R.A. - spend 3000 yen on Arms Micron at (Yamada, Yodobashi)

The shield gimmick is clever.