Or does it simply mean they are four steps above the base trans newtonian level?
Three steps, because TL1 is base TN tech level. And yes, it means exactly that. If your engineers find modules, they'll be at TL4 (e. g. magneto-plasma engines, ultraviolet lasers, delta shields...).
I noticed today something that I may have missed before. A team unearthed some ruins and the details of the race were "Aristocracy of Pediese -TL4". Now I am guessing that last part is short for "technology level - 4", if that is so what does that mean? Is there somewhere I can see what any given technology level represents? Or does it simply mean they are four steps above the base trans newtonian level?