Hi yong,
I have confused with the previous solution, T4 defines this meta tag via this file:
/plugins/system/t4/themes/base/html/layouts/t4/index.php
To override this file, you copy it and past into this directory of the template:
/templates/t4_blank/local/html/layouts/t4
create this path if it doesn't exist on your site. Then edit the maximum scale to the value as you wish.
Regards