*******************************************************************************************
goto http://rxtx.qbang.org/wiki/index.php/Download 
   and download rxtx-2.1-7-bins-r2.zip from "Binary" column.
goto C:\Users\Downloads\rxtx-2.1-7-bins-r2.zip\rxtx-2.1-7-bins-r2 
   and copy RXTXcomm to 
   C:\Program Files\processing-1.5.1\java\lib\ext
goto C:\Users\Downloads\rxtx-2.1-7-bins-r2.zip\rxtx-2.1-7-bins-r2\Windows\i368-mingw32
   and copy both rxtxParallel.dll and rxtxSerial.dll to
   C:\Program Files\processing-1.5.1\java\bin

*******************************************************************************************
The result:

Stable Library
=========================================
Native lib Version = RXTX-2.1-7
Java lib Version   = RXTX-2.1-7
[0] "COM4"