The smallest four digits are 1000.

1, number. 2, a few; Some. 3. days; Fate. 4. The basic mathematical concept of expressing the quantity of things. Due to the need of counting and measurement in production practice, natural numbers are first generated, and then fractions, zeros, irrational numbers, negative numbers and imaginary numbers are gradually generated. 5. Grammatical category. Indicates the number of things that nouns and pronouns refer to. 6. refers to mathematics.