People need to keep in mind that Etymotic is not about pushing the limits of audio performance. The company is about listening accuracy and hearing protection. They really couldn't give two thoughts about so-called 'audiophiles'.
Don Wilson is an employee of Etymotic. He doesn't control what products get rolled out. If there's anyone that has a vested interest in Etymotic and the direction of the company, it's Mead Killion. I highly doubt he cares about maximizing the performance of the ER4 over something that may potentially be of societal benefit, such as the EtyKids and its accompanying iPhone app.
We can safely assume that Etymotic will never create an IEM with a 'colored' response. Every single one of their IEM efforts since the ER4 has been to recreate the ER4's response with lower cost materials and manufacturing.
But yes, there can easily be improvements to the ER4 design, and it doesn't even need to be with a dual-BA setup. Mead Killion used to work for Knowles. He likely knows all the people there (that matter, at least). He can easily commission a single-BA with the correct diaphragm stiffness and sufficient back volume to yield a frequency response with even higher accuracy capability. The ER4's biggest sonic problem (in my opinion) is ultra-low frequency roll-off. That can be corrected with the right venting of back volume. High frequency deficiencies can be rectified by introducing well-placed perforations within the diaphragm. Better vibration reduction can be done by designing a dual driver with the same armature to back volume ratio as the ED. Perhaps the slightly larger housing would even spur them to design a more ergonomic housing (not exactly their strong suit).
However, if the ER4B is able to achieve 92% accuracy (this is based off Killion's own older study, which may not be the final word in the best possible averaging of diffuse field eardrum responses amongst the general populace, as not enough peoples' individual HRTFs have been measured), then that's good enough for Etymotic. Why spend many more R&D dollars into proprietary-spec drivers for an extra 3-5% in accuracy? Business-wise it doesn't make sense, as it doesn't feed into the company ethos of protecting peoples' hearing.