1 is OK but it doesn't explain that it's uncomfortable. It's also ambiguous because "hard" could also mean "difficult".
2 definitely sounds as if you are saying that you find it difficult to sit down on, or remain sitting on, the chair.
3 is better (you don't need "it" at the end) but also doesn't mention that it's actually uncomfortable.
4 has given me a great mental image and is probably the best one for suggesting the chair is uncomfortable but it's also very specific about the problem that the chair causes.
The simplest way to say what you want to is:
This chair is uncomfortably hard.
You don't need to include "to sit on" - that is a chair's function. You don't need to remind the reader that people sit on chairs.
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