What a sticky situation.
Unfortunately in this day and age you would have thought that people would be more accepting of interracial dating. It seems as though the more things change the more they stay the same.
Here you are, you have found a girl that you have a great time with, who obviously feels the same way, but because her parents don't approve your relationship becomes a lot more complicated.
I have run into similar situations before as it is important to my parents that I meet and marry a guy that has the same religious background as I have been brought up with. Until I met my current boyfriend, I had never dated a guy with the same background. It caused numerous fights and arguments with my parents and it made one relationship in particular, very hard.
The fact of the matter is that unfortunately our parents tend to have a very strong hold on us in every way. I know that for me, although the fights were always because I was defending my ex, a lot of times I went away and ended up thinking about what my parents had said and slowly their feelings started to make sense and I could sometimes even understand what they were saying and where they were coming from.
Remember that She is Young
I think its also important for you to remember that your girlfriend is also only 17. She is still young and vulnerable. While four years may not seem like a big age difference, 17 and 21 are . Your ideas of what each of you want out of the relationship as well as how intense it will be may not be the same.
First of all, you need to talk to her.
Encourage Her to be Honest with Her Parents
Explain that you know that it is her parents that are making things difficult but encourage her to be respectful and honest. If she is not honest, your relationship becomes based on a lie or deception- that is no basis for building a good solid relationship.
Also, I think it is important for you to think about the fact that ultimately she may have to choose between you and her parents- personally I don't think that is a fair choice to have to make and at no time should you give her an ultimatum.
This is sure to be difficult for all the people involved but in the end, the final decisions lie with her. Its tough. The only advice I can give you is to be as supportive as you can be.
It is unfair that her parents are putting your relationship under so much pressure but its reality - a harsh reality that you shouldn't ever have to deal with. Keep your head up, think good thoughts and hopefully in the end her parents will be able to see you for who you are rather than for the color of your skin.