Well, having #4 here has worked out well so far. Instead of three boys playing and there always being an odd man out, they have paired up and played (mostly) well together.
#2 & #4 went fishing earlier today, but spent more time getting themselves wet than their lines. That kept them busy for about 2.5 hours.
#1 & #4 rode their bikes to Burger King for lunch, then went to the air museum. They were occupied with that for about 3 hours.
It has been quite peaceful here today. Hopefully this will continue!