I'm going to guess the Lenovo W520/W530 or a Dell Precision Mobile Workstation. Those are the major laptops I know of that come with 4 DIMM slots (32GB now, 16GB last year because 8GB DIMMs were crazy-expensive).
Currently sat at my w520 and it's a brilliant piece of kit. I've always been a thinkpad fan - although I can appreciate the draw of the MBP.
It's little touches like the spill-proof keyboard, carbon fibre reinforced plastics, trackpoint, and amazing thermal management (even with an i7 in there the bottom never gets hot, only mildly warm).
