You can just scale it here (most of the time, anyways, depends on how it was done):
First, you have to find the exact page, or scene where an explosion is shown as a feat. Depending on the environment where the explosion occurred, you may have to scale the size of a smaller object that is shown in the map. Let's do an example... An explosion of unknown size occurs in this...
vsbattles.fandom.com
Anyways, assuming 4km diameter, which means 2km radius, and if it was on ground, it'll get around 600~ kilotons. Aka High 7-C+.
If it was on air, it'll get a bit lower, but still High 7-C.
For 4km radius, then it'll be Low 7-B.
Not even sure how you got this number ngl. What calculation did you use?