I'd imagine it's limited somewhat, but with the crossfire bridge it shouldn't be too severe of a performance hit. I've only seen a few articles test 3870s in Crossfire, and none that tried to limit the bandwidth to only 4x. Here's an article that shows the performance hit with some other cards in single card configurations, so I'd assume the hit would be even smaller with Crossfire enabled.
http://www.tomshardware.com/2007/03/27/pci_express_scaling_analysis/