Немного запутанно, но можно так:
select
locate('<h4 class="notify__title notify__title--small">', html, 1) as o,
locate('</h4>', html, locate('<h4 class="notify__title notify__title--small">', html, 1)) as c,
concat('<span>',
substr(
html,
locate('<h4 class="notify__title notify__title--small">', html, 1) + 47,
locate('</h4>', html, locate('<h4 class="notify__title notify__title--small">', html, 1))-locate('<h4 class="notify__title notify__title--small">', html, 1) - 47
), '</span>') as new_html
from test;
SQL test