Even if he did have a parasite that he was managing (usually ich, most others aren’t managed effectively, for that long anyway) 4 years is a substantial time to go without adding to the genetic pool for that parasite. It has been said that ich can live for up to four years without the introduction of a new ich strain- meaning eventually if you aren’t adding new parasites, the existing ones will start to fizzle out... naturally.
There’s a good chance that even if you were managing ich at one point... with 4 years of no new additions, it would not be present now. If someone is managing ich actively, I’d still recommend QT so that they aren’t bringing in new strains to refresh the gene pool, or worse- velvet, brook, or uronema.