How about Using random Numbers?
Private void spawn()
{
Int Count = 4;
Int Random = 0
If(Count>0){
random = greenfoot.getRandomNumber(x,c)
If(Random=1){
...
Count--;
}
Etc.
}
Will this be during gameplay? if so, what parts should be random? possibilities are (a) when one is spawned (b) which one is spawned, (c) where one is spawned.