I used to beam my wifi over 2 km out to the ice fishing shack. I used a pair of 800 mW transceivers with yagi antennas. I believe 4 km is is the limit for a useable link due to protocol timing requirements, but I'm sure it could be at least detected at greater distances.
I talked with a military tech one evening, he also mentioned operating at 4 km. This guys house was easy to spot because of all the antennas. Any houses like that within a couple miles of your place?