Hm, I'd be careful - this means you have to run with a default route and you'll want to be absolutely sure the default route pointing to your upstreams depends upon the upstream being being able to carry the traffic!
Ie, if you have two ISPs A and B, and your default points to A, then you're hoping that A will actually be able to reach all the destinations which you're filtering announcements for. There's the possibility that A will suffer a transient failure and not be able to "see" parts of the internet - you'll pass them packets to destinations you don't have a route for (and hence use your default) and the ISP A won't be able to do anything with them.
Another more common issue is your default ISP going away - ie, you default to ISP A, ISP A disappears and BGP detects it, but the link stays up and thus you send all your traffic into a blackhole. There's a couple of ways around this. The easiest way is to use SLA tracking which can be configured to ICMP ping the host every few seconds to see them being up, then make your route dependant upon that monitored object being up.
The feature is called "Reliable Static Routing Backup Using Object Tracking". I have no idea whether its supported on the Catalyst 4500; you'll have to do a little digging.
