There is a free version for Linux called pioneers. To add 3 computer players need to open new terminal and type:
pioneersai -n comp1 -s 127.0.0.1 & pioneersai -n comp2 -s 127.0.0.1 & pioneersai -n comp3 -s 127.0.0.1 &
found at http://ubuntuforums.org/showthread.php?t=1359342