I have tried ignoring it. I've tried time outs. I've tried removal of toys/privileges. I've tried gently explaining why he mustn't do it. I've tried telling off. He knows it's wrong and always says sorry, but then does it again the moment he doesn't get his own way.
His sister never did this. We've never used smacking as a punishment, nor has anyone in the family. His dad's the gentlest man on the planet. Just now he had a tantrum about brushing his teeth, and hit me several times on the legs and chest. When that failed to provoke a reaction, he hit me twice in the face. Brushed his teeth for him and put him to bed. Told him he wouldn't get a story as he'd hit me. So he hit me again. 

Other than this - fairly new, it's definitely post-3 - behaviour, he's a sweet and loving boy, who's very friendly and has good manners. Is it just a phase I need to ride out, or is there something I should be doing?