I think the Audi A8 MAF is what you're looking for. I also did a couple searches on "vw 100mm MAF" and there's quite a few other solutions out there too.
*EDIT* Appears the A8 MAF is 3.5", and 4" MAFs are all custom aluminum joints. Only other info I found is from TBT-Synchro and a few others running a Pro-MAF.
http://promracing.com/mass-air-meters.html






