If you had inhuman strength previously, it would have cost you 2 refresh. If you buy or upgrade to supernatural, you pay an extra 2 refresh for a total of 4.
If you are starting with supernatural, you pay 4 refresh total.
If you later upgrade from superb to mythic strength, you pay an extra 2 refresh for a total of 6.
When you have an upgraded version of a power, you ignore the lower version because it gets replaced with the better version and you default to the rules under the upgraded version.
So, once you have supernatural strength, you no longer use the text under inhuman. Just use the rules under supernatural.
Toughness and speed powers also work this way.
Some powers might have caveats(although I can’t think of one off the top of my head) so it might say, “in addition to power ‘x’ you also get xyz.” Incite Emotion powers do this. You start with one power but can pay extra refresh to upgrade it for extra damage and range. In which case you pay for all the combined upgrades.