It's really not a matter of GL4 vs GL5; but rather the additive package used. I've been running GL5 in all my Triumph diffs for many years now, and it works great. But, I chose a full synthetic GL5 (Valvoline Synpower) that doesn't use the nasty "active sulfur" additive that can erode "yellow metal" over time.
I can't prove it, but I believe that any oil rated both GL5 and MT1 should do fine for the diff. MT1 includes a long term, high temperature stability test that the "bad" additive can not meet.
Before I switched to MT-90 (which IMO is worth the price, I bought a case on eBay to get the per-quart price down); I ran Valvoline VR1 (racing) 20W50 motor oil. It worked well enough (the ZDDP helps protect the thrust washers, which are a known weak spot in these gearboxes), but the MT-90 works better in terms of easier, crisper shifts.