I have a similar script that keeps a fixed number of children running at all times. Basically, you just set a variable for the number of kids out, then have the main script decrement that by one when a child exits (however you do it - I use signals, but see the caveats above). In your case, you need to have some code like this:

## Not real code! Very poor pseudo-code actually. :)
$kids = 4;
$kidsOUT=0;

$SIG{CHLD} = \&ReapKids;
for (1..$kids) {
  FORK or die
  if (child) {
    Do child things
    exit
  }
  else {
    $kidsOUT++;
  }
}

sub ReapKids {
  ## This detects when the kids are finished, somehow
  ## If signalling, verify the PID as well
  if (KidHasDied) {
    $KidsOUT--;
  }
}

{ ## Second loop, runs when all kids from first are finished
  if ($KidsOUT) { ## Still waiting!
    sleep 10;
    redo;
  }
}

## Same as above...spawn however many kids you need...
[download]

This is vey, very, rough code but should illustrate one way to do it.