A hard-coded string enclosed in single quotes is known as a string literal. An example is 'Hello world'. Another example is the very long string you are trying to insert in the table. By contrast, 'abc' || 'def' is a string expression but it is not a string literal. Similarly, to_char(sysdate, 'yyyy-mm-dd') is a string expression, but not a literal. "Literal" means constant, hard-coded text.
The issue you are facing has nothing to do with insert, or to_clob(), or the data type of columns in your table, etc. It only has to do with the string literal itself.
In Oracle, a string literal can be at most 4000 bytes long (or 32767 bytes if the database is set up with extended MAX_STRING_SIZE). PERIOD! There is no way around it.
So, the question is, how can you ever get a string as long as the one you have into a table with a CLOB column. The answer depends on how you are receiving the string in the first place. The best option would be if it came in chunked already - as a collection of strings, with a tag (an id) to keep track of which fragment belongs to which CLOB and an ordinal number (to show if it's the first chunk, the second, etc.) Then you could re-assemble them using TO_CLOB() on the first chunk, plus the concatenation operator.
If your process is to type 72000 characters at the keyboard, you will have to type 4000 of them at a time, enclose in single quotes, and use the concatenation operator (essentially doing by hand what I described above). You would also have to use TO_CLOB() on the first fragment (otherwise the concatenation will fail).
Another option is for the string to come as a value, from some application that supports long strings (something compatible with Oracle's CLOB) and that can hand over such values to the Oracle database without the need to write out the hard-coded string in full.
So, the ball is in your court. The first question is, Where is the long string coming from in the first place?
