On the CAT4500, channel member ports cannot span multiple switches. Meaning you cannot have say Gig1/1 on 4500A and Gig 1/1 on 4500B in the same etherchannel going to say a 3750
On a CAT3750, if you are using stackwise, etherchannel members can be on any interface in the stack
The following page has samples for 3550/3750 and a 6500 but is similar even on a CAT4500
http://www.cisco.com/warp/public/473/89.html