Thanks for your comment. Hopefully, I can buy a used minibox card from a member here. I sold my F111 to my friend at work but I still keep the er4s. I guess the cost of modding the er4s with an upgrade cable terminated with balance connector will be around the price of the card. Could be fun.
I am doing the same thing to my ER4S, I have all the parts here but haven't gotten around to doing it yet. I have S adapters from Awwan on eBay, a replacement Westone cable, and a TRRS connector. The only soldering i have to do is the TRRS connector. No foolery with resistors and such.