those 4 and 8 double A holders are notorious for bad connections, and they bend easily compounding the problem. I have a 4aa holder here for rx and servo troubleshooting that is wrapped in 3 directions with zip ties to make contact.
a rx pack would definitely work. or you could get out the voltmeter and troubleshoot your 4aa holder.
You did have the dark color on the battery plug to the outside of the rx case, right
