You could uncheck option Formatter -> Oracle Formatter Options -> Comments -> Align the right margin, but that would still give you the following:
SELECT *
FROM DUAL -- Comment1
WHERE x = x -- comment 2
AND x = x -- comment 3
AND x = x;
-- comment 4
The thing is that formatter want to obtain ...
SELECT *
FROM DUAL
WHERE x = x AND x = x AND x = x;
... but the end-of-line comments are in the way. So the next best thing it can do is to output them and continue the actual code on the next line.
One could think of moving the comments forwards or backwards but I guess that might open a can of worms.
Andre