Learn Finnish: Liuoksesta haihtui yöllä reilut neljä prosenttia. - A good four percent of the solution evaporated at night.

This is a special rule for measurement approximations. When you use adjectives like reilu (a bit over) or vajaa (a bit under) right before a basic numeral in the nominative case, the adjective takes the plural form (reilut, vajaat). The number itself remains singular.

If reilut is plural, why is the verb haihtui (evaporated) in the singular?

The actual subject of the sentence is the numeral phrase neljä prosenttia. In Finnish, subjects consisting of a number (other than one) followed by a partitive word always take a singular verb. Adding the plural modifier reilut to the beginning doesn't change this fundamental rule.

Does reilu always mean 'fair' or 'generous', as it does in everyday speech?

It does originally, but in the specific context of numbers and measurements, reilu means 'a bit more than' or 'a good [number]'. So reilut neljä prosenttia means slightly over four percent. Its opposite in this context is vajaa (a bit less than / just under).

Why is the word for 'solution' in the elative case (liuoksesta)?