Are those 2 comments related??!
I have just asked my German secratary about the football, as she has young kids who play, and apparently as a general rule, they take kids who have already started school, but if your son is reasonably advanced for 4, you won't have a problem. They just like kids who are old enough to actually focus a little & follow instructions. If you know where in Frankfurt you will be living, I can find out where your local clubs would be easy enough? Just PM me.
Rounders, my goodness. I've not even thought about that game in over 20 years! At my school, all the boys who were crap at cricket had to play rounders with the girls instead. Not much of an incentive to take up cricket as their games were always shorter & gave you more free time anyway. And you got to meet girls.