Dark fiber and LAG across the two links. VPLS won't work on 4500.
Disable STP on the DCI link (and make sure nobody ever reconnects the cables the wrong way) or use MST with each DC as a region (you don't want STP events in one DC to spill over into the other).
In theory you could use Metro Ethernet services, but if they don't pass LACP and STP transparently, you're in a total mess (read: don't even try).
Also, please note that there is no safe way to stretch layer-2 networks. Layer-2 network is a single failure domain - you get a loop somewhere, everything gets affected. You can limit the damage by using storm control, but then you're killing the good packets with the bad.
Finally, think about routing and connectivity implications if the link between the two data centers fails completely.