GL5 fluids contain EP additives that may be harmful to brass elements in transmissions calling for GL4 fluid.
That's the way it used to be. The old fashioned extreme pressure additives to make GL-5 were damaging to brass & bronze. GL-4 oils had a milder add pack that didn't harm these yellow metals.
Modern GL-5 additives do not harm yellow metals. If the label on the jug says that the gear oil is recommended where either GL-5 or GL-4 are spec'ed tells us that this oil is OK. And, not all GL-5 gear oils use these modern additives, so read the label.