You are a minor still, it's illegal for someone of your age to have sex with a 19 y/o . You really shouldn't even be dating him, however if you really like him why don't you get to know him more. Wait till your 18 then consider having sex. .
Generally speaking, no. In most states, there's a 3 year window for age-gap relationships. Additionally, you may want to think carefully. A 4 year age difference at your age is enormous, because the two of you are in pretty different stages of life. It would be near impossible for the relationship to be balanced because so much life experience happens between 16 and 19, so it is likely that he's going to be making most of the decisions and controlling what happens... and that isn't going to be healthy for you. I know that may not be what you want to hear. I hope, though, you'll at least give some thought to it.
I know in Australia, the legal age of consent is 16, but obviously didn't countries have different laws.. I think it would best to wait till you are 18 before anything sexual happens between you both, just so you are on the safe side.
You would not get into trouble, but your friend could go to jail - it's that serious, and there's a good reason for the law being that way. Because, he has much more life-experience than you, and the relationship would be unequal and exploitative. Depending on the age of consent in your country, it might or might not be ok for you once you turn 16 however, but still, you 16, him 20...still sounds a bit exploitative to me, but that's just my opinion, some might disagree. And in some countries / jurisdictions, you would need to wait till you are 18. Might be a good idea to find out exactly what the law is in your case.
If his parents did not approve, even without proof of any sexual contact, I believe his parents could charge you with statutory rape which would then require you to be a registered sex-offender and that could be potentially life ruining.
It depends where you are in the world, but in most juridstictions, 15 is below the age of consent, so any sexual activity could land you in trouble. I also think you would be wise to take Chip's comments about the four year gap into consideration - you are both at very different stages of maturity and development.
