Lots of us on here have, I think, had good experiences with the pull-along bikes! I have a Burley Piccolo because I am freakishly short and do not have enough seat post for attaching, but others have used that kind with great success.
I would not buy a trailer for a four-year-old unless you think there are more tots on the way. My kids are both big fans of the Piccolo and even my small-for-his-age kid was able to ride it at age four. (He could not pedal at that age but he can now that he is five.)
Many people prefer the seatpost kind but I do like that I can still use panniers (with great caution) with the Piccolo as it attaches to a cargo rack.