Thobela is building a pyramid of matches. The first section uses 400 matches, the second uses 550, and the third uses 700. How many matches will she need for the fourth section?

The number of matches used for the first three sections are 400, 550, and 700.
Calculate the difference between consecutive terms:

$$550 - 400 = 150$$ $$700 - 550 = 150$$

The pattern shows a constant increase of 150 matches for each subsequent section. This is an arithmetic sequence with a common difference $d = 150$.

To find the number of matches for the fourth section, add the common difference to the number of matches used in the third section.

$$\text{Matches for 4th section} = \text{Matches for 3rd section} + \text{Common difference}$$ $$\text{Matches for 4th section} = 700 + 150$$ $$\text{Matches for 4th section} = 850$$

Thobela will need $\boxed{\text{850 matches}}$ to complete the fourth section.