Both ways could be correct. As an isolated sentence, without context, it would be best to say it is correct without "the". However, in a context where both the speaker and the listener know about these specific publishing houses, then "the publishing houses" becomes more possible. It depends on what's come before during the conversation and before the conversation. For example, they could be the only four publishing houses in the area or they could be the publishing houses that are the speaker and the listener's clients. In general, if you have a difficult time deciding whether or not to use "the" - definite article, ask yourself if the noun you are considering qualifying with "the" is something specific. Is there specificity?