Soldato
I don't know about the 4 pot, but the M54 has 2 different types of tensioners and of course I ordered the wrong tensioner pulley so I still have the squeak squeakInteresting, just did a search and it sounds the same:
So I should replace both idler and tensioner pulley I guess. According to the comments the OEM is good for 65k, makes sense since I've had the car from 42k.
Edit*
These two are what I'd need yup?
http://www.gsfcarparts.com/parts/engine-parts/belt-drive/auxillary-drive-belt-tensioner
It's a pretty easy DIY if you don't want to pay the indy, took me about 10 mins.