I got one legendary from the first tier 3 lockbox I opened.
I got my second Legendary from the first tier 4 lockbox I opened.
Both were the same exact Rocket Launcher
I'm amazed at how lucky some people are, I am at 850 EGO and have opened dozens of lockboxes and yet I"m still using a 186 EGO Green shotgun and a 236 EGO Blue Assault Rifle and a 286 EGO Blue shield
Also you do NOT get a guaranteed legendary from your first t4, thats just lies.
"Boohoo it takes time for legendaries"