Focal started making plug and play speakers for BMW and here is one with the correct bracket.
fronts:
http://www.abt.com/product/81005/Focal-IFBMWS.html
rears (non-HK hence no tweeter)
http://www.abt.com/product/81007/Focal-IFBMWC.html
under seat subs:
http://www.abt.com/product/81001/Focal-IFBMWSUB.html
The specs seems inferior compared to their standard build quality... and you're probably paying for convenience. There is literally no wire splicing or anything... a true plug and play.
However, a 4" mid is easy enough to get into the opening with a fabricated bracket.