If he doesn't know the modern cute version of anpirate, why dress up? Doesn't seem like it will mean anything to him.
For some insane reason we had shown ds POTC at 3* and he had his own way of dressing as a pirate. (I love little boys' imaginations...they so rarely need an official costume at all) So if we had gone on a cruise like that when he was 4 it would have been very self-led for him. Otherwise we wouldn't have dressed him up for it. And we still don't, finding the bandanas
dcl leaves for everyone for pirate night to be perfectly adequate.
*he had already met Sparrow at
Disneyland without knowing who he was and then again that same year after watching he movie and had great interactions each time.