4th port? My device has only 2 USB ports. I suppose it is different. It might need modified dtb, but as long as it is s805 (and it looks like it is), I would expect it to at least boot. We would need to debug, what uboot does.
The most straightforward way to do it, would be to open the case and connect UART interface (e.g. ft232). Dumping the full environment (printenv in uboot console), would give us some insight.
There is alternative way to get the dump, with fw_printenv, but I haven't ever tried this and don't know, if this is possible on the stock software.