The esc is setup with the battery packs in series just like the 1st picture Rusty posted. Personally I'd break out the soldering iron and get rid of one of the connectors but you can buy/create an adapter as well.

If you want to maintain the ability to run 2 packs, simply get a female deans connector and solder a single piece of wire to both leads of the connector to create a jumper. Plug it in and now that connector is bypassed allowing you to run a single 4s pack. Unplug it and you can use two packs again. Found these images by googling "deans jumper" so I assume you can find them on eBay or whatever using the same terms.