I don't think there any physical limitation for communicating with a probe at a couple light years at least. Project Daedalus had this all modeled and specified back in the 1970s (https://en.wikipedia.org/wiki/Project_Daedalus), using a 40 meter directional antenna on the probe.

Not to mention sufficiently big megascale engineering not to mention some of the more crazy kinds of propulsion (NSWR, antimatter, etc.) should be perfectly visible with current technology over 4 light years and possibly much farther away.