Which sentence is punctuated correctly?

don wore braces on both of his legs still; he was an avid runner.
don wore braces on both of his legs: still he was an avid runner.
don wore braces on both of his legs still, he was an avid runner.
don wore braces on both of his legs; still, he was an avid runner.

The sentence that is punctuated correctly is:

Don wore braces on both of his legs; still, he was an avid runner.

This is the correct option because the semicolon is a punctuation mark that is used to separate two major sentence elements, just like the ones presented here. Moreover, there is a comma after still, because it is an introductory phrase and the author wants to emphasize its sense of contrast.