I don't think virus is a 4th declension noun, and I've seen it described in multiple sources as a 2nd declension (meaning slime, poison, infection, mucus, or something thereabout). It is unusual in being a -us noun despite being neuter, but irregularities happen (other examples of neuter 2nd declension -us nouns being vulgus and pelagus). In classical Latin, it is a mass/uncountable noun, and only got the modern meaning (and the ability for form plurals) in recent times.
That said, your point about virulent vs virolent is interesting, but I don't know enough about sound changes, or how that word came to be and made it's way to us to know if this is counter evidence to being 2nd declension, or if it can be explained away otherwise.
references: https://en.wikipedia.org/wiki/Latin_declension#Virus https://en.wiktionary.org/wiki/virus#Latin https://gaffiot.fr/#virus
That said, your point about virulent vs virolent is interesting, but I don't know enough about sound changes, or how that word came to be and made it's way to us to know if this is counter evidence to being 2nd declension, or if it can be explained away otherwise.