Cathar's guide to fan shrouds:
http://forums.overclockers.com.au/sh...ad.php?t=49874
Mentions (post #21) that adding an internal baffle to separate the chambers between fans increases the airflow. This suggests that 4 separate 120mm shrouds might be better, as the fans are not fighting against each other.

This thread:
http://forums.overclockers.com.au/sh...=shroud+baffle
Suggests a 20% increase by using a baffle (which seems high to me!).

EDIT: yeah this info is old school... but the general principles shouldn't change, and I haven't seen any more recent testing.