Learn Russian: Четыреста рублей — это мало.

You probably learned that 2, 3, and 4 take the genitive singular (so 'four roubles' is четыре рубля). However, round hundreds like 100, 200, 300, and 400 take the genitive plural. This is why we use рублей here.

Why does четыреста end in -ста instead of -сто?

For the numbers 300 and 400, Russian combines the first number (три or четыре) with ста, which is a special form of сто (hundred). So it literally translates to 'four of a hundred.'

Why do we need the dash and это in this sentence?

Since Russian doesn't use a present tense verb for 'is,' a dash is often used to link the subject to its description. The word это acts as a pointer, so the sentence literally reads 'Four hundred roubles — this is little.'

Could I use an adjective like маленький (small) instead of мало?

No, because маленький describes physical size (like a small box). Мало is an adverb of quantity meaning 'not enough' or 'not much.' When using это to give your opinion on an amount, you must use the adverb form.