"The firm faced four tough contract negotiations last year that threatened to divide employee loyalties and forced the carrier to cancel some flights."
I would use only the second sentence. Apparently you can use the first one too. I think it's just a matter of idiomatic usage and not so much about the noun being countable or uncountable. One additional note, many uncountable nouns can be used with "a", but to me, that still doesn't mean they're truly countable. There are countless exampless of this usage.