Hi, I just had this setup installed. Here's what I suggest.
1. Just let the techs install and hook up one of yr computers to the Comcast modem.
2. When they are gone, plug the WAN connection for the 4100 into the Comcast modem (I told em this was what I was going to do, they didn't care)
3. Reboot the Comcast modem (mine has a battery backup so I had to push a pen in the hole in the back -be careful not to press for too long as you may reset to factory settings, just a quick click worked for me)
4. Plug your computers into the 4100 LAN ports
5. If you need more ports, just buy a switch (I use NetG#$# GS108) and plug a network cable between a 4100 LAN port and the switch - then you can plug computers, printers, etc into any spare port on the switch or the 4100 and they will all get IP addresses via DHCP and talk to each other.
You should not need to clone the MAC address.
The 4100 should work on the stock settings unless you have VPN, webcams, servers etc.