Career SLG 4.000? Sure, it's possible. OOTP just proved it.
Looking through my retired players list, I clicked on sort by SLG. Why, I don't know, because you can have some very high career slugging percentages left behind by coffee cup players.
But a final career SLG of 4.000? Sure, it's possible. What is required is that the player have only one career plate appearance and that he hit a home run in it. Nothing else whatsoever.
So it did happen in my league. I will treasure this fellow forever. My imagined backstory:
He was a big mouth; came up and hit his home run, took too much time going around the bases; cavorted and capered too much; one of his teammates took him to task; the two battled in the dugout; the manager got between them and took a fist to the jaw for his efforts; the team owner witnessed all this; and Jon Boyce was released the very next day, never to be heard from again.