All great advice so far. We have two kids (5½ & 7½) both of them are already taking an interest in the water. The rules on our boat are this, pfds are ALWAYS on while we're out sailing, no negotiating on this.
Try and keep your 4yo engaged as much as possible with sailing terminology as well as water safety. I sometimes even make ours tell me port or starboard when we're at home....lol
Also, we try and keep our sailing short and sweet as to not let them get bored.
My 7½ yo is just learning the wonders of the Opti....She's going to sailing camp and loving every minute of it! Hoping she'll be ready to crew on our H28 sooner than later
That's just humble advice from a sailing dad.