Seriously, is this normal behavior in a parent, or am I just being too sensitive?

Maybe he just thought that it was a little rude....my dad says that I am sometimes, but he never slaps me. I don't think you're being too sensitive. I also don't think that it is right of your own dad to slap you 4 times. Maybe he has anger issues? I think you should talk to someone about it. Hope this helps!