Multiple Processors/Threads use with ffmpeg

[altsysrq]

I am using Subsonic in a virtual machine that has 4 cores. Unfortunately, streaming a 1080p movie does not utilize more than one core from that machine. I tried enabling multiple cores with ffmpeg using the "-threads #" option without any success. I tried both 0 and 4 as an options and I get an error immediately during playback.

The message is:
Video not found or access denied: /stream?pathUtf8Hex=<lots of random numbers here>&maxBinRate=1000&timeOffset=0&player=1

Here is an example of one of my changes on the transcoding page:
ffmpeg -ss %o -i %s -threads 4 -async 1 -b %bk -s %wx%h -ar 44100 -ac 2 -v 0 -f flv -

Does anyone have any idea what I am doing wrong? Is the threads switch in the wrong location?

[altsysrq]

Ok, 0 and 1 does seem to work (contrary to what I posted above - not sure how I mixed that up). Anything else results in a failure.

I also tried compiling ffmpeg-mt and replaced ffmpeg, but got the same results.