1) Malware is a program that does something bad, but you let it. For example, a program that asks "Click here for a fun program" and you click it and it erases your hard drive. That's malware because it's bad, but it's not a virus because you had to click "ok" whereas a virus would just do it without asking.
2) Not yet. Possible, in theory, but a lot harder than writing malware to trick people with, so that's what people try instead.
3) Hard to say, usually there would be some kind of negative effect you'd notice. Macs have had some malware that actually don't DO anything, they just exist to prove that they can exist. So in that case you may never know!
4) YES YES YES. If you don't know what people are doing to your computer, you could have anything happen!