You May Be a Jerk
If you picked answers 1, 2, or 3, then you are a courteous person.
If you picked answer 4 then you are a jerk. I'm sorry, but there is just no way around that. When you use your cell phone in a public place you are being boorish and imposing and inconsiderate—in other words a jerk.
(And if somebody tells you that your conversation is disturbing and you respond, "Sorry, I didn't know this was a library." then you are a flat-out asshole. Really.)
There are several reasons why cell phone discussions are more disruptive than conversations with people. They all have to due to feedback and cues.
The first issue is that during a phone conversation, you don't have the visual cues that indicate your discussion partner is receiving you. When you have those cues you will modulate your voice level to the point where the conversation registers, but not too far beyond. (That is, unless you are one of those unfortunate people that through some tragedy of genetics or upraising never acquired an "inside voice.") You don't get those cues when talking to a non-seen person on a telephone, so you are naturally going to raise your voice to a level where it's sure to be heard.
The second issue is that the old "Western Electric" phones used to feed a little bit of the microphone signal back to the earpiece. That way you could hear yourself and adjust your voice level accordingly. Modern phones don't do that, thus taking away another cue that helps you modulate your voice level.
Try as you might, you just can't avoid being disruptive when you use your cell phone in a public place. So next time the phone rings in a restaurant, please consider taking it outside. And be assured that your phone won't explode if you let the call roll to voice mail and return it later.