How to start a screendump with AppleScript?

I tried to start a screendump with the command line

keystroke "4" using {shift down, command down}

however I failed choosing the correct process to target with this command.

Can somebody help me with that???

Model: G4 /400 AGP
AppleScript: AppleScript D1-1.9.3
Browser: Netscape/7.2
Operating System: Mac OS X (10.3.9)


it’s easier to use the shell command screencapture

do shell script "screencapture -i ~/Desktop/picture1.png"

is almost the same as shift-cmd 4