How is 4908 configured in this scenairo. AFAI understand we are just extending over connectivity to other site using the ISP's L2 backbone and using the dot1q tunneling.
What you have to do is just configure the port coming from the ISP side to a 802.1q trunk and configure the respective routers to a particulaer vlan.
On the other side ISP will use 802.1q tunneling to assign a dedicated vlan ID on which you both the vlans 501,201 will be carried till it exits the ISP network. On the exit interface the traffic will be segregated again as 201 and 501.
-amit singh