Jason,
The PIX can do that.
I would suggest using one address for the PIX outside interface. Use 3 other addreeses to create a NAT pool (the last one should be used for NAT overload or PAT).
Save one of those 5 IP addresses for testing (i.e. to connect a PC to the connection in front of the firewall).
Liberty for All,
Brian
Brian Ford | brford@cisco.com | brford@yahoo.com | 51 75 61 6c 69 74 79 20 6d 65 61 6e 73 20 64 6f 69 6e 67 20 69 74 20 72 69 67 68 74 20 77 68 65 6e 20 6e 6f 20 6f 6e 65 20 69 73 20 6c 6f 6f 6b 69 6e 67 2e | Email me when you figure this out.