4MBPS would be disappointing, as this is roughly what the 3GS stores 480p at? would imply that little more detail was being stored ( or that the encoder was a LOT better ) I'd hope for at least 8-10MBPS personally to retain any decent kind of quality.
edit: seems flip HD is about 9MBPS so I'd probably expect it to be right about there, higher still would really be preferable, but as a tradeoff it's probably generally OK.