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The 2811 doesn't really have the performance capacity to handle its two built-in FE interfaces (let alone four). If your interfaces are passing enough traffic, oh say 20 to 50 Mbps, that might be what is account for your high CPU.
The 2811 and other ISRs were generally designed to handle the bandwidth of a WAN interface.
PS:
This link, http://www.cisco.com/web/partners/downloads/765/tools/quickreference/routerperformance.pdf, will show an old Cisco performance sheet for various routers.
