howto set maximum nodes?

How can I ensure that a job requesting 4 cpus lands on a node with (at 
least) 4 free cpus?  What I'm seeing now is the job going to 3 cpus on 
node1 (which has 4 cpus but a 1 cpu job already running on it) and 1 cpu 
on node2 (and node2 had 4 free cpus).
PBS 2.3.16
maui 3.2.6p9

What keyword should I look for in the manual/FAQ?

Thanks,
Chris
--
Chris Evert
chris@xxxxxxxxxx
Geophysical Development Corporation
Houston, TX