[4 Watt FM Transmitter]

Hi Cerium,
You must get rid of TR4 and R2 and the rest of the mic preamp so it doesn't load down your external input.
Don't you want the stereo input to be combined into mono like this?:

Got a mod for the 15W version to do stereo input?
http://electronics-lab.com/projects/rf/014/index.html <-- this 1.