I agree that your gut is a powerful barometer. I can add that at this age--and certainly at 4--excessive interest in/focus on sexuality (you're right that some can be "normal," even when very young) is tied to the child having been over-stimulated and exposed to sexual content, whether in the form of overt abuse or covert abuse (anything from showing a young child materials/media concerning sex, talking to him/her/them excessively, letting the child bear witness to sexual encounters).

In other words, your fiance's son may be a victim of abuse or premature exposure (versus being a "pervert," which wouldn't really mean anything).

If this child can be brought to a therapist, a professional would be able to evaluate.

If that's not possible, then can you discuss openly with your fiance?

If nothing else, protecting your own child is key, as you already know. But it will entail a lot of watching and the problem may worsen as puberty approaches/goes on for your fiance's son.

I feel for you. Good luck. Trust yourself.