Hello again, Navi,
I've been philosophizing about the general issue here, which is, I believe, one that has come up in other threads of yours -- namely, the issue of whether an infinitival (adjunct) clause of purpose can be retained once the main clause to which it belongs is passivized.
So, looking at (3), if we take an active version of the same sentence -- "She kidnapped him to scare the other dissidents" -- the sentence is perfect. But when the main clause is passivized, as it is in (3) ("He was kidnapped to scare the other dissidents"), the result becomes questionable or incorrect.
Above I said that I found (3) incorrect. Upon further reflection, I see why I found it possible to hear (3) in such a way that it was not incorrect -- even though I didn't mention that I could hear it as not being incorrect. It ties in with your use of the term "dissidents." That is to say, I think content matters.
The word "dissident" is used in institutional or governmental contexts, in which agency and purpose are depersonalized. We see the depersonalization in the vague use of "they" in active sentences: "They
[whoever they are] kidnapped him to scare the other dissidents." Notice that in the active version two paragraphs above I had used "she."
My point, however, doesn't concern gender. I think the crux of the issue is whether the action is institutional or personal. If it is an institutional action
, then its purpose (assuming it has one) will likewise be institutional -- one might even say disembodied. If, on the other hand, it is a personal action
, its purpose will be that of one or more identifiable agents.
The way this plays out at the grammatical level is that, whereas the subject of a infinitival purpose clause relating to a personal action needs to be clear, the subject of an infinitival purpose clause relating to an institutional action need not be clear. This has clear applications to passive voice:
5a) He turned on the television to relax.
5b) The television was turned on to relax. *?!
6a) They drank the water to quench their thirst.
6b) The water was drunk by them to quench their thirst. *?!
7a) They / the city installed stop lights to alleviate traffic congestion.
7b) Stop lights were installed to alleviate traffic congestion.
8a) They built the bridge to facilitate commerce.
8b) The bridge was built to facilitate commerce.
The action of kidnapping can be either institutional or personal. I think that you are aware of that, Navi, and that that is why you gave kidnapping a personal motive in (1) and (2), and an institutional motive in (3) and (4). By varying the semantic content of the infinitival clause, you gave the action of kidnapping a personal-institutional aspect shift and raised this very important point.