The GL-4 has a few more additives for anti-rust, anti-foam, etc. which might not be absolutely necessary for use in a transmission, but "nice to have". The main concern is to avoid the extreme pressure additives the GL-5 has which can attack bronze parts such as synchronizer rings. I used to do a lot of business with a Fuller Roadranger transmission rebuilding shop in San Francisco when I ran a truck shop out there. They had 55-gallon drums full of trashed synchronizer rings caused by people running hypoid gear lube in Roadranger transmissions.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!