The answer of your question is in your question itself
"Original project was done in flex4.0 and it uses some swc libraries"
before this para it was said
flex4.6 works normally in debug
It should be working fine with 4.6 because with this version playerglobal.version 11.1 being used normally which has JSON lib included with it but prior version to playerglobal - 11 you need to include "as3corelib.swc" in your project libs.
More details you can find at - http://blog.infrared5.com/2011/07/working-with-native-json-in-flash-player-11/
Thanks,
Varun