If I am 17 about to be 18 in 4 months is it illegal for me to date a 21 year old?

Depends on where you live. The laws vary by location.

When in doubt, ask your parents.

How did you both meet ? Would your parents approve ?

Here in GA for example, we had a man arrested last month, for interfering with custody, girl was over age of consent to have sex but parents ordered her and man not to see each other, so when adult man, tried to date a minor child, the DA proceeded with the custody issue. There has been many prosecuted on that in GA.

There are no laws governing dating. But you may be under the age of consent so having sexual could be a big issue.

Why not just be regular friends for another 4 months and keep getting to know each other better. Then, if 4 months later you still have strong feelings for this person, then you can start a relationship.