No no. Babylon 4 originally was going to sequel Babylon 5 after it's destruction. According to what I read a while back, and what I recently re-read on wikipedia, the idea was for Babylon 4 to be a totally different station, and would sequel the timeline of Babylon 5 for 5 years. But they scrapped the idea and put into the past instead of the future. I guess it was like Babylon 5 came first, then Babylon 4, Then 3, then 2, then 1? But Babylon 4's original name was "Babylon Prime". So, who knows where that could have gone if that was the case. I think it's canon to assume that EarthGov was building another Babylon, or had one built, in the case of Babylon5 being destroyed. I assume the Minbari would have been aware, as well as the Vorlons, that if the other station were destroyed, thier chances of ascending from the Rim would be slim. The First Ones ascended because they acheived the one true purpose of thier species: to make more species.