Add this to the end of your pickUp function.
if ( DNA1.x==5000 && DNA2.x==5000 && DNA3.x==5000 && DNA4.x==5000){
completeScreen.x = 512;
completeScreen.y = 384;
}
This should work, but a better way would probably be to add some Boolean to check agains instead like these.
var DNA1Collected:Boolean = false;
var DNA2Collected:Boolean = false;
var DNA3Collected:Boolean = false;
var DNA4Collected:Boolean = false;
Then as the player collects them you can set them to true. That way down the road if you want to do something besides move the x to -5000 your if statement will still work.