I have explained this multiple times There is no way to test and certify an oil as GL-4. Its obsolete and the prjmary test equipment (new factory prepared Dodge truck axles) are long gone.. You have no idea whats in any of those "Gl-4" oils or how they perform. API no longer recognizes any claim of GL'4 performance.
GL-5 oils ars actually tested, certified., snd recognized by API.
You might want to ask the sellers of those GL-4 oils your liability question. I doubt you will find one that will accept liability in any specific application. Thats becayse yhey can't backup their GL-4 claim with a ASTM STP 512A certification test report. All they csn say is they deem it equivalent in most applications. I can show you beaucoup certified GL-5 products that make that same claim. And BTW - the oil you linked is actually a GL-5 oil
Now if want to get technical we can look at the now obsolete STP 512A test requirements for GL-4 and GL-5 service. What you will see is the only difference is the wear tests. GL-5 out performs GL-4 and thats why API never updated the requirements tests for GL-4.
From my latest round of questions to AMSOIL Technical Services on the GL-4 to GL-5 compatibility with Kubota axle and gear oil "recommendations", I wish that I was equipped with the information that we have now on this firecracker of a great thread. I just may when we ever get an rainy day here in the deep South, call them back up and see if they will send me a copy of that acknowledgement of the compatibility from Kubota as they kept refering to in my questioning. I'll bet that it will be a miracle if they send something to me on that, but if they do I'll post it here for sure.
In the meanTime, the GL-5 in the ZD mower gearbox is running like a champ, even in this heat, and to some degree I think it's running cooler and there is a little less venting off of the pressure vent valve compared to the GL-4 lube oil. I'm keeping my eye on this one close as this will help a lot of good folks out there trying to make that informed decision. I know that this really helped me despite the fact that I have been messing with oils and lubricants in all kinds of heavy manchery my entire life. Get it wrong and you will pay the hefty price though!
Eric