High speed microcontroller choice
FireWire pretty much rules out anything on the market that's usually considered a microcontroller. Also, FireWire is certainly not what I'd call a &quot;simple serial protocol&quot;.
4 MB/s and SD card as writing target is yet another indication that you need a device with sufficient RAM for buffering; so, again, not a microcontroller.
I don't think FireWire is common anymore in SoCs integrating a sufficiently powerful application processors, so quite likely you'll either want a device that has PCIe and is able to run a mainstream operating system (Linux?) that has support for a commercially available PCIe/FireWire bridge (if such a thing still exists), or you'll want an FPGA SoC and someone with extensive digital design experience to implement a FireWire controller in the FPGA. It might still be, by far, easier to let a normal operating system running handle the SD card, filesystem and buffering.
So, sorry, not a microcontroller's job, it seems.