Welcome to tsf. You can only crossfire identical gpu's. Meaning it must be a 4870 with a 4870. You can't crossfire any crossfire compatilbe gpu's with eachother, they must match.
And yes they do make 6 pin to 8 pin adapters. However I'm not sure if they are universal.
http://www.pcconnection.com/IPA/Shop...ci_sku=8708329
http://www.thermaltakestore.com/8pin...1--p86112.html