Yes, according to posts here by BMD representatives, and the current user manual from BMD's support site, the BMPC-4K is designed to work with conventional 1080 HD-SDI devices and also 4K capable 6G-SDI devices.
Of course, there's always a chance a particular HD-SDI device might not be compatible with the BMPC-4K. We'll have to wait for the cams to ship before we know for sure.
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