A 4% signup rate actually signifies really well targeted emails. Not everyone who doesn't sign up is annoyed by the mail (or did even open or read it).
I think this is far from beeing spam, where you would see signup rates well below 1%.
"Email spam, also known as junk email, is unsolicited messages sent in bulk by email"
If you've never spoken to this person before, you have no business relationship with them, and they have not opted in to receive mail from you, you are an email spammer. You can mince words if you have to to sleep easy at night, but this behavior is wrong.
The author of the article did something shady and you are defending him.
Then only options I see is to a) hope google finds you interesting enough, b) paid advertising, or c) word of mouth.
What are the other methods ? I am asking because I can't think of anything else.
1) If the mail you receive actually interests you, you wouldn't classify it as "spamming" ("Did I really sign up for that? Anyway it sounds good"). If not you may avoid them in the future, but you would not have bought their product anyhow.
2) Scale: If a (real) spammer sends out 20 million emails and 0,0001% buy viagra. Thats fine for him. The other Emails cost him close to zero, so every sale it profit.
Of the two above, the latter is probably the spammiest, and might technically count as "spam" since they're unsolicited, but they're generally (in my opinion) targeted sufficiently well enough to be acceptable/tolerable. In excess, though, they can quickly cross that line into being unquestionably spam.
You're spamming 25 people to get one sign up? That sounds terrible.
SPAM is defined by the recipient, not the sender. Thus, if you're trying to justify something as "not spam" by saying it meets or doesn't meet some criteria, then it's probably SPAM.
If an email is an email about a thing or service which is related to my interests, but I don't recognize the company, I suppose that falls under spam, but that's better than random emails for prescriptions or scams.