Which of these is incorrect and why?

Question

1. It is cheap to replace these chairs.
2. These chairs are cheap to be replaced.
3. These chairs are cheap to replace.
4. These chairs are cheap to replace them.

To me, 1-2-3 seem to be correct. If not, what is wrong with them and why?

4 seems to be incorrect because of 'them' - unnecessary repetition... Although 'replace' is usually followed by an object, the phrase 'cheap to replace' refers to 'These chairs', so not necessary to use 'them' again... Is it so? If not, why? Could you give a more precise grammatical explanation?

Finally, which of these seem to be most natural? I guess 1 and 3. If not, why?

2 is correct but sounds kind of clumsy... Or maybe it's perfectly OK?

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@Sander and @Omnidisciplinarianist

Thank you for taking the time to answer ^_^

To sound a bit more logical semantically, let's change 'cheap' to 'expensive'.

Both @Sander and @Omnidisciplinarianist, I think I get your point about the linking verb 'be', and yet the logic seems to me elusive.

In sentence #1 & #4 'is' and 'are' respectively appear to have completely identical role. In #1 'these chairs / them' doesn't refer to 'it', so the sentence sounds all right. Both of you seem to consider the whole phrase 'expensive to replace' as the complement. But if we consider the complement to be 'expensive', then 'to replace' needs an object and in #1 it has its object. So, is the whole thing really because of the linking verb 'be'?

Next @Omnidisciplinarianist and partially @Sander, #2 and #3 are both correct and I would say identical in meaning, though #2 may not be the best choice. Intuitively I can't but come to the conclusion that 'to replace' in #3 sounds almost as passive as 'to be replaced' in #2, though practically it's not. Could this observation be explained in solid grammatical terms? If yes, obviously a passive verb (or a verb 'behaving passively') would not take a direct object.

Finally, @ both, let's slightly change 4 in this way '5.These chairs are too expensive for us to replace them.' Now I still think that the sentence is wrong because of 'them', applying similar logic as above, but I'm not as sure as I would be about #4. I mean if I'm reading through a text and see #4, it will stick in my eyes, if I see #5, I think I'll overlook it. What are your thoughts about 5? Maybe you think it's correct? If yes, why?

Again @Sander and @Omnidisciplinarianist

I had some further thoughts about the sentences... Sorry if you feel it's boring to discuss this question more :(

1. It is expensive to replace the chairs.
2. The chairs are expensive to replace.
3. To replace the chairs is expensive.

All of these three are correct, we agree, right? OK, so we just rotate the infinitive phrase (or whatever you call it grammatically) around 'be expensive' - putting it at the back in #1, splitting it in #2 and putting it at the beginning in #3. If we use 'them' after 'replace' in #2, then when we bring 'to replace them' at the front in #3 it is more than obvious that such a sentence would be incorrect - 'To replace them the chairs is expensive.' Maybe in some special context the following may be correct 'To replace them, the chairs, is expensive.' What are your thoughts about this reasoning?

Answer 1

You are correct that the first three sentences are grammatically valid.

You are correct that the fourth sentence is incorrect. As @Sander states, the copular verbs glue the rest of the sentences together, all but that final them. This leaves the them referring to something that isn't actually there, and thus is said to be dangling.

With respect to which roll naturally off of the tongue, you're correct that both #1 and #3 do; I've heard both forms in casual conversation.