I like the kid trailers just fine, but I would go for the ute in your case. The other option I would consider is a Burley tag along bike. Your 4 year old will be looking to contribute to pedalling and there will be a gap of several years between wanting to pedal and being able to keep up. I have used a Burley Piccolo on several of my bikes and it is much easier to cope with than the other tag-along bikes.A full cargo bike (ute, other longtail, front loader) gives you the option of carrying an extra kid if one of yours wants to pick up a friend.