- I have never seen such a beautiful beach like this.
- This is the most beautiful beach I have ever seen,
- No other beach is as beautiful as this (one).
- I have never seen anything like this beautiful beach.
I think sentences 1,2,3, mean about the same, but what about 4?
Is 4 correct in the first place? [. . .]
If correct, does it mean the same as the other three sentences?
The only incorrect sentence in your example set is (1). "Like this" doesn't work there; however, if you changed "like" to "as," or omitted the phrase altogether, the sentence would be fine. You could also use "so beautiful a beach" instead.
(1a) I have never seen such a beautiful beach as this.
(1b) I have never seen such a beautiful beach.
(1c) I have never seen so beautiful a beach.
As to (4), though correct, it does have a different meaning. It means what it says, that this is something unlike anything that the speaker has seen before. It isn't a question of beauty. Perhaps this the first time the speaker has seen a beach!