Drove from Uluru to Coober Pedy and back in December last year. Unfortunately most sign pictures I have were from SA, not NT. As with the rest of outback Australia, I recall all paved roads being posted with route numbers, though the locals referred to them by name. IIRC there was a change in numbering when crossing the SA/NT border. In SA, it was numbered "A87" on signs, so that would make it N87 or just 87 in the NT, though I don't remember which one it was or whether it was both. Photos from my April 2010 drive through the NT show all highways being numbered without prefixes. In the Outback it seems numbers come and go, but names are forever.
IIRC NT-4 was labeled exclusively as just 4, and I don't recall seeing any "B4" on any signage. No data to add on what is "supposed to come." Only a confirmation that this was numbered, but I suspect GSV could tell you that too.