Can I use these two sentences below interchangeably?
Is the sentence 3 a reduced form of the sentence 4 ?
In both cases, I'd say yes, for most purposes.
Rightly or wrongly, there is a subtle snobbery to thus, because of its old-English sound, which can sometimes be used to imply that the outcome is in some way obvious. It's not wrong to write this, but you may need to consider whether the outcome of the action is guaranteed.
Consider, "They have developed a new technology, which they expect will allow them to reduce costs," to soften the guarantee, if you do not intend this air of obviousness.
Comment: Could you answer my other question " Why I can't use this form "conjunctive adverbs + v-ing" with other conjunctive adverbs but I can use it with "thus"." and give your opinions on the sentence 5 and 6
I don't see anything wrong with sentences 5 and 6. Both are reasonable, although the consiseness of 5 may be preferred in a written document.
I also don't see anything wrong with using conjunctive adverbs in the manner that you have described, although there are some conjunctive adverbs that would fit your specific sentences better than others – it really depends what you want the sentence to mean and whether you need it to have positive or negative implication.
- The oil producers will raise prices, thus increasing their profits... – their profits will increase as a direct result of raising prices
- The oil producers will raise prices, therefore increasing their profits... – again, their profits will increase as a direct result of raising prices (but note my earlier comment about the air of obviousness)
- The oil producers will raise prices, moreover increasing their profits... – the profits will increase as well as prices. This phrasing is a bit awkward, though – in fact, I would probably avoid moreover entirely.
- The oil producers will raise prices, however increasing their profits... – I wouldn't use this one, but it could technically be used to mean their profites will increase in spite of raising prices. It would feel more natural to use this one with a negative initial statement, e.g. The oil producers' costs will rise, however increasing their profits due to increased prices, but this a really awkward way to express this sentiment.
If you are specifically looking for academically-sound answers, I can't really help here – I only speak English, I didn't study it.