If you have a 128MB stick with just 4 devices this has to be 256Mb chips in a x16 organisation. And in a x16 organisation they have the same addresspace as 128Mb chips in a x8 org. Therefore it is not difference to the chipset.
You are right, that normally 256Mb chips are not recogniced by older chipsets. But in this config it shouldn't be a problem.
I havn't seen 128MB Dimms based on 256Mb devices right now. I assume the 4 device stick is quite new. And especially i didn't knew, that Micron is offering this kind of DIMMs already!