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For load balancing, across two devices, you have several options, from simple to complex. If you're working with gig links, that aren't really going to be fully saturated, a simple approach might be sufficient. You also need to decide whether load balancing based on outbound traffic is sufficient, or whether inbound load balancing needs to be considered independently of outbound. (I.e. if inbound traffic is in response to outbound traffic, if you balance outbound, inbound, generally, balances correspondingly. But, if inbound traffic is being the initiated, e.g. to public servers you're hosting, then balancing that traffic is much more complex.)
For example of simple outbound load balancing, if you can route your outbound traffic to both your edge routers, "equally" (GLBP, ECMP), you'll roughtly load balance outbound, and if inbound traffic is in response, to two different NATed IPs, it will roughtly load balance inbound.
Next layer of complexity, if you're carrying fully Internet routing tables from both your ISPs, and if you exchange that between both your internal routers (iBGP), then each of your internal routers will direct traffic to the other internal router if the other has a better (shorter Internet AS path). This assumes you're still initially sending outbound traffic to both routers "equally", as described in the prior paragraph.
The next layer of complexity might be to use Cisco's OER/PfR technology. This technology will load balance, dynamically, on actual link loading and/or measured performance to end destinations. This technology, I don't think is available on 4500s. Full technology is available on many of the software based routers, unsure whether the ASR series is included.
Again, if traffic is initiated from the inside, inbound traffic will generally "track" your outbound balancing. PfR also supports dynamic inbound balancing, but that's a much more complex setup and you might need cooperation from your ISPs to obtain maximum effectiveness.
