crikos - I bought a 3.25" to 3" reducing 90 degee elbow. 3.25" over the tb and reduces to 3" at the bend. this allows a fit to the 3" maf housing (or whatever combo of 3" couplers and joiners you use to get to the maf housing); it also provides more clearance from the back of the engine bay.
homeboy - pretty tight in there, huh? i'm currently using the larger of the bmc air boxes with a 3.25" filter. even that doesn't fit well. I bet 4" is tight. one thing that will gain you some room is to remove the insulation on the inside of the engine bay on that side - should buy you 1/2" or so.
another option might be to not mount the filter in parallel with the engine, but rather have the piping do a 90 degree turn into the actual side intake and mount the filter element in the fender well. they are doing this on caymans with a kit from fabspeed:
BGB / Fabspeed Cold Air Intake Install (with Pics) -
you would still probably have to go with a smaller filter, however; I think there would be room in the fender well, but the opening into the engine bay is only *i think* around 3.5".
otherwise, i think you could diy something similar very easy: remove the rubber liner in the fender well, trace the inside flange onto a piece of lexan, drill a 4" (or whatever) hole in it, bolt it in place, mount the air filter through the hole and connect the piping.