I'm not a lawyer, and you need one for this. You're right that the issues can seem murky. But--in general--how much you can use depends on what percentage of the work you use (which in itself depends on how long the work is). That's one reason why poems and song lyrics are so tricky. Four lines can be a violation if it's the full poem, for example.

Also, the prominence or position of the lines in the work can matter: for example, whether you use the lines in the text or as an epigraph for a book or chapter. Again, I'm NOT a lawyer, just broadly familiar with some related issues. So I'd encourage you to get some legal advice on this one.