I started my research in 09/04. Last year I was considering of quitting, but I was convinced to continue. The feeling of quitting came back again, and it feels stronger. Since I don't intend to pursue a academic career, I constantly evaluate the purpose of a PhD. I love literature, but do I like it that much to devote 4 years on it? Career-wise, are companies (publishing, media, pr) more likely to hire someone with 4 years of work experience with Master's degree, or someone who has 1 year of work experience but has a PhD?

You are significantly more employable with a PhD and receive promotion quicker. For longer term benefits, you should stick out the PhD.
You have the chance to get this done and dusted..... you may regret quitting later on.