I need help... four flow meter to arduino mega.

Projects Programming
8 
  attachInterrupt(IRQ_B, CounterOUT, CHANGE);

and

void CounterOUT()
{
  if (digitalRead(FlowB) == LOW)
  {
  pulseCountOUT++;
  }
  if (digitalRead(FlowB) == HIGH)
  {
  pulseCountOUT++;
  }
}

Why do you define the interrupt to react on changes and then check for low and high states? Especially given you eliminate the second part of the handler as you noted in your last post.