The painting is too beautiful to have been painted by a mediocre artist.

The correct answer is 4 as the given sentence is correct, no change.
The sentence given is a combination of two sentences that have been combined with the phrase 'too ... to' to form a simple sentence. 'Too' means 'more than enough' in the underlined expression, referring to the elegance of the painting, which is more than enough because it was painted by a mediocre artist.
The sentence has the proper 'too ... to' form, so the sentence given is right, option 4 is correct. This is a basic phrase containing the adverb 'too' that replaces 'too' with 'so...that' when it is translated to a complex form.
If we choose option 1, we will need to edit the sentence slightly more by putting "that" followed by a negative clause, as in "so beautiful that it could not have been drawn by..." We are not allowed to change this statement to complex, thus option 1 is incorrect.
Option 2 is incorrect because it implies 'as much as that,' which does not finish the sentence because it conveys a positive meaning while the sentence has a negative tone.
Similarly, 'very' emphasises the beauty of the image; regardless of what the word says, 'very' is a positive adverb, whereas 'too' has a negative sense because it refers to an abundance. Option 3 is also incorrect.