This should work, but maybe the rtmp playback on the mp4 file is failing? Since these are already files, can you try this by accessing the files directly? Something like this:
fullBinaryPath=./evo-avconv arguments=-i /path/to/videosource4recording.mp4 -i /path/to/Kalimba.mp3 -c:v copy -c:a copy -map 0:v:0 -map 1:a:0 /var/evostreamms/media/OutFile6.mp4
This will eliminate any issue with the network pull. If this works, then we can look into why the rtmp pull is failing.
If this fails, can you send me the two files so that we can try to replicate here?