Hi,
I was going to say that the ARE 18F parts with 4 USARTS available (18F67J94 for example) but, as you obviously already HAVE done, I checked the device support list and PBP does not support them....
An alternative approach might be to use an external UART chip, interfaced to the PIC thru the MSSP module. Just a quick Google search turned up the MAX3107 as an example but there will be others I'm sure.

/Henrik.