The indefinite article in English, ‘a’ or ‘an’, is very simple. The German  equivalent, ein, not only means ‘a/an’ but also ‘one’. The numeral ‘one’,  when counting, is eins, e.g. eins, zwei, drei, but when followed  by a noun ein in both senses takes all the above endings, the distinction in  meaning between ‘one’ and ‘a/an’ being made by stressing the former, and  the ending being determined by the gender and case of the noun, e.g.

Sie hat nur e i n e n Sohn.
She has only one son.

Sie hat einen Sohn.
She has a son.

There is of course no fourth column with plural forms in the above  paradigm, unlike for der/die/das, because by dei nition ‘a/an’ cannot be  followed by a plural noun.