Learn Russian: Мы смогли работать вместе с четырьмя поставщиками. - We were able to work together with four suppliers.

Четырьмя is the instrumental case of the number четыре (four). We need the instrumental case here because of the preposition с (meaning "with"). When you do something "with" a certain quantity of people or things, the preposition с forces the number into the instrumental case.

I thought numbers 2, 3, and 4 were followed by the genitive singular. Why is поставщиками in the plural?

That rule (using the genitive singular for 2, 3, and 4) only applies when the number is in the nominative case, or when it's the inanimate accusative. When the sentence forces the number into an oblique case (any other case, like the instrumental here), the noun simply matches the case of the number and goes into the plural. So, "with four" is с четырьмя, and the noun must be instrumental plural: поставщиками.

Why do we use the perfective verb смогли alongside the imperfective verb работать?

Смогли (were able to / managed to) is perfective because it focuses on a single successful result: they successfully achieved the ability or agreement to work together. However, the action they managed to do—работать (to work)—is an ongoing process or a general state, so it stays imperfective.

Why is there a soft sign (ь) in четырьмя?