Hi,
When you have connected the 4'th pc to the l3 switch, were you able to ping the other pc's successfully.
Were you able to ping your default gateway?
You have mentioned that there is no entry for the 4'th pc. Where were you checking the ARP table entry ? is it on your L3 switch?
If so, the problem might not be anything related to your service provider. We have to check your lan...
-VJ