How to properly attach a "tag-along" bicycle
You mention four bolts, does your seatpost hitch look something like this:



By design, these can cinch up pretty tight against all size seatposts, in contrast to two bolt models which need the proper size shims:



However, with the four-bolt there are some caveats:


Do you have a carbon or aluminum seatpost? With both, you have to be careful as it's possible to crush the seatpost if you tighten the bolts too much. In these cases, you might want to ask the help of a bike store - they might give you some carbon fiber seatpost compound that will help with friction, as well as tighten the bolts using a torque wrench to prevent seatpost collapse.
If you have a steel seatpost, it's next to impossible for you to tighten the bolts too tight. If the bolt heads are stripped, get new bolts at your hardware store and get good quality steel ones (note: bolts with socket cap heads are stronger than the bolts with the button cap shown in the photo). Be sure to tighten in an X-pattern, just as if you were tightening the lug nuts on a car tire or the bolts on a head gasket. Rather than an allen key, you might want to use a hex bit in a socket wrench, you can get much more torque:





With the proper amount of torque on the bolts, it really shouldn't twist under normal circumstances. You could drill a hole to pin it, but that would weaken your seatpost.  Instead, you may try using some blue loctite on the bolts to make sure your bolts are not backing themselves out and a bit of the same friction compound used in carbon fiber bikes on the seatpost to add just a tad more friction.


p.s. When buying second-hand, always make sure your product isn't subject to a CPSC recall such as this one: https://www.cpsc.gov/Recalls/1998/CPSC-Riteway-Products-Announce-Recall-of-Bicycle-Trailers