He could ride his bike only so far.

The correct answer is 4 as the given sentence is correct, no change.
Options 1, 2, and 3 are incorrect as 'just that far' implies that he could not go to a specific location,' just as far' cannot be used as 'as' should be good and then gives a different interpretation as well, and 'only far' cannot be used as relative pronoun is missing. He has only been able to ride his bike so far.