Short answer:
The things that are created in these two ayat are different. 41:10-11 refer to the creation of the earth (ard) and the things upon it, which is done in 4 days, whereas 41:12 refers to the creation of the seven heavens (samawat), which is done in 2 days. So altogether they add up to 6 days.
There are also other ways these ayat are understood, beyond a literal interpretation of "days".
Long answer:
"Day" is usually taken to refer to a time period in general, rather than a "day" in the sense of one rotation of the earth since our measurements of time rest on the physical world, and there was no such thing as an earth-day before the creation of the earth and sun. That said, a narration ascribed to Imam al-Baqir (A), in response to an enquiry about how a "day" could be meaningful before creation, says that time is also measured in an area of existence near Allah, and therefore is not only reliant on the physical world.
Some people maintain that the expression "two days" (or "two periods") refers to spiritual and physical aspects of creation, or that "days" refers to distinctly different stages of creation, rather than timespans.
Some people also understand "four days" with respect to the earth to refer to the four seasons, insofar as they are equal in length and relate to sustenance.
For instance, in Tafsir al-Qummi, "two days" is explained as "the beginning of Creation and its end", and "four days" is explained as "the four times when Allah brings out the world's means of sustenance... spring, summer, autumn, and winter."
If one is taking a numerical approach to "days", however, Qur'an 41:10-41:12 outline the creation of our world as follows:
41:10 - the earth (ard) in 2 days
41:11 - mountains, sustenance, and blessing in 4 days
41:12 - the heavens (samawat) in 2 days
Elsewhere, the Qur'an mentions that the world was created in 6 days.
From this, it can be understood that the 4 days in 41:11 include the 2 days mentioned in 41:10. That is, the earth and its features were, altogether, created in 4 days. Then, the heavens were created in 2 days. Altogether, these sum up as 6 days - or, rather, spans of time.
