I want to buy a car, but there is very high interest on loan. I'm also thinking about leasing a car, or buying a used car with finance or even buy a new car for $4000 (this is all I have got right now).

Check www.craigslist.org of your area and buy from owner..That way it will be cheaper take someone who knows about car with you when you go to see. Its not worthy to buy new car or old car with loan money or lease.

I am guessing you didn't mean you want a brand new car just for $4000 (although your post sounds like that's what it means). If you meant you want a new car and are willing to put $4k down, then that's a different thing.

If your budget is limited, I suggest not to get a brand new car. Yes, you will get financing but the terms will be ridiculous - your monthly payment will be too high. Remember, you have to pay for tag and registration so keep aside a grand for that. Then go get a reliable second hand.

Craigslist and advise :Never go alone and with money and his place, always choose a public place. Always bring a person who knows about car.
You should not go with looks and cleanliness only someone must see under the hood as well underneth the car. Pay a service fee for mechanic advise. If you can run a carfax , is always better. Even a minor problem will cost you over hundreds because mechanics' nick name is butcher.
My passanger's side window moter is not working and I got a quote for $200 from a Brazilian Mechanic. Motor costs 55 and time to put on is about Half an hour. If I take it to a dealer will be over 500. Watching a lot youtube and thinking of giving a shot (after one practice in Junkyard).
Long time ago my clutch did not come up, I asked my co-worker and told me what to do. I did not get what he was suggesting. Then he told me if you do not know a b c of car better to buy a new one.