The charge sounds forceful because it treats Quran 41:9-12 as three separate additions: 2 + 4 + 2. The stronger reading, found in the classical commentaries and supported by the Arabic wording, is that the "four days" includes the first two, so the total remains six.
Why the objection seems strong
The critic is not inventing a random problem. The Quran says in several places that Allah created the heavens and the earth in six days:
Indeed, your Lord is Allah, who created the heavens and the earth in six days, then established Himself above the Throne.
Quran 7:54
And again:
He who created the heavens and the earth and what is between them in six days, then established Himself above the Throne: the Most Merciful.
Quran 25:59
Then Quran 41 says the earth was created in two days, its mountains and provisions were set in four days, and the heavens were completed in two days. If those are three separate blocks of time, the critic says, the total is eight. That is the strongest form of the objection.
What Quran 41 actually says
Say, "Do you truly disbelieve in the One who created the earth in two days and set up equals to Him? That is the Lord of the worlds." And He placed on it firm mountains above it, blessed it, and determined therein its sustenance in four days, equal for those who ask. Then He directed Himself to the heaven while it was smoke and said to it and to the earth, "Come, willingly or unwillingly." They said, "We come willingly." So He completed them as seven heavens in two days and revealed in each heaven its command.
Quran 41:9-12
The key phrase is "in four days" in verse 10. It does not have to mean "four more days after the two." It can naturally mean: by the end of four days, the earth and its preparation were completed. In other words:
Earth itself: 2 days.
Earth with its mountains, blessing, and measured provisions: 4 days total.
Seven heavens: 2 days.
Total: 6 days.
This is how major commentators such as al-Tabari, al-Qurtubi, and Ibn Kathir explain it. They did not see an arithmetic problem because the language does not require adding the four to the previous two.
The ordinary language point
This kind of speech is not strange. A person may say, "I built the foundation in two months, and completed the house with its utilities in six months." No one has to understand that as eight months. The second number can be cumulative, giving the full time for the broader stage.
The wording in Quran 41 fits that pattern. Verse 9 mentions the creation of the earth. Verse 10 expands on what was placed in it and how its provisions were measured. So the "four days" covers the earth-stage as a whole, not an added independent stage.
What this does and does not prove
This answer does not prove every wider question about cosmology, nor does it depend on defining "day" as a 24-hour day or as an epoch. The arithmetic issue is separate. Whether "day" here means a divine period, a stage, or something not identical to our solar day, the internal count is still six stages on this reading.
What it does establish is more limited and more precise: the alleged contradiction is not forced by the text. The critic's total of eight depends on reading "four days" as "four additional days." But the Arabic, the flow of the passage, and the classical explanation all allow - and strongly support - reading it as "four days in total" for the earth and its provisions.
So the passage is not a plain numerical contradiction. It is a case where a quick addition overlooks how the sentence is structured.