Yes, this is a layer 3 feature. WCCP uses IP redirection to achieve transparent redirecting to a proxy server.

One way you could accomplish this, if you really want to keep the majority of routing in your checkpoint firewalls is to create an external transport network on the outside of the firewalls and add one hop through a layer 3 interface on the 4510 before handoff to your ISP.  You would have to either have your proxy server outside the firewall (probably not the best idea) or allow special access back in for proxy hits. Of course this complicates your configuration because you need to follow certain conventions to share a device between security contexts like that securely.

My personal preference would be to let the ckeckpoint devices focus on firewalling and bring routing back into the 4510, but this may not be a good fit for your scenario.