In case anyone's interested,
Cyberguys has on clearance a 120mm Panaflo fan very similar to the one used for our packs, except it has a tach output lead, is rated 7V - 13.8V (might not do much at 6V, let alone 4.5V), 0.46A (from 5.52W at 12V rating) vs. 0.6A, and a "hydrowave" oil suspension -based bearing. 2500 rpm, 103.8 cfm, 41.5 dBA.