How do I know that min-term can't be combined any further?

Question

I'm trying to learn (and implement) Quine-McCluskey algorithm for boolean function minimalisation. I'm learning the algorithm from wikipedia example.

From that I understood the following:

1. Take all 1s and don't cares from table and put them in tiny table, ordered by number of 1s in the variables.
2. Any two rows that are only different by 1 digit can be combined. The diferent digit doesn't matter then:

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 m8     | 1000
 m9     | 1001
 -------------
 m(8,9) | 100-

Now here, the wiki also states that the expressions that can't be combined any further shall be marked by *. But in their example, why can't m(4,12) = -100 be combined any further? Can't I combine it with m(8,9) = 100- producing --0-?