#!/bin/sh

startNewProcess() {
   echo Starting process $1
   sleep 10 &
}

for file in 1 2 3 4 5 6 7 8 9 10
do
   while true
   do
     [ `jobs | wc -l` -lt 4 ] && break
     jobs
     sleep 1
   done
   startNewProcess $file
done
wait