I just got off the phone with AMD/ATI tech support. He stated that the 4670 does have the bistreaming capability. He also stated that the 4670 is a higher performance card vs the 5470. He said the way to tell is from the 2nd digit of the model number (4"6"70 vs 5"4"70). He said from 1 - 4 is a low end card, 5 - 7 is midrange, 8-9 is high end. When asked what the difference between the 4000 and 5000 series was, he stated the 5000 series offers:
smaller form factor (i.e. they further miniaturized the chip)
energy efficiency (the 5000 series has capabilities to reduce power consumption when idling)
Support for DirectX 11 (4000 series only supports 10.1)
basically you would need a 5670 or to equal the performance of the 4670 and get the additional benefits of the 5000 series.