James Fergusson, a British architectural historian and writer, mistakenly attributed the stupa at Sanchi to 'Tree and Serpent worship' due to his lack of understanding of the significance and history of the site. This misinterpretation was a result of his limited knowledge of Indian religious and historical practices, and he made this erroneous claim in his writings on Indian architecture.

In reality, Sanchi is an important Buddhist archaeological site with a rich history dating back to the Mauryan period. The stupa at Sanchi is a Buddhist monument, not a center of 'Tree and Serpent worship.' Fergusson's mischaracterization of Sanchi's significance has been widely criticized and corrected by later scholars who have provided a more accurate understanding of the site's historical and religious importance.

Therefore, the correct option is [4].