Hello there,
It would be easier to change one of the link rather than adding a new option. For ex. to change the Foursquare link to Instagram
<?php if(sketch_get_option($incart_lite_shortname.'_foursquare_link')){ ?><li class="foursquare-icon"><a target="_blank" href="<?php echo esc_url(sketch_get_option($incart_lite_shortname.'_foursquare_link','incart-lite')); ?>"><span class="fa fa-foursquare" title="Foursquare"></span></a></li><?php } ?>
just change the icon class and title attributes of span tag (this change will only be reflected in the front end, not in the back end).
<?php if(sketch_get_option($incart_lite_shortname.'_foursquare_link')){ ?><li class="foursquare-icon"><a target="_blank" href="<?php echo esc_url(sketch_get_option($incart_lite_shortname.'_foursquare_link','incart-lite')); ?>"><span class="fa fa-instagram" title="Instagram"></span></a></li><?php } ?>
Regards,
Tanay Khandelwal