Hi Amit,
Indexing is different from using serial number. Usually indexing counted in the order of the devices connected physically to the PC where as serial number of the FT4222 remains same.
There is a separate software and hardware device from FTDI to write the serial number to a fresh FTDI device. I couldn’t recollect the device details off my head right away but you or your team may know that very well from hardware designer point of view. Using that external device and application from FTDI, you may write the serial number to each devices. You serial number could be lengthy alpha numeric.
Once you have the separate serial number for each device, you will initialize it through the same APIs to get it connected with separate sessions.
The under hood of the library is password protected for secret of sauce. However, since these initializations are taken care by original FTDI DLL libft422.dll , I can confirm you that under-the-hood will not give much of clue in this case.
I hope this can help you explore further. Please feel free to get back to me of any more help.
Regards,
AJ