Can I get intimate with a guy who is four years older than me when I am younger than 18?

Assuming this guy is over 18, the legality of it would depend on where you live. In some areas, it's illegal for anyone over 18 to have sex with a minor; in others, there are laws that protect couples that are close in age when one or both of them are underage. Practically speaking, however, you should be extremely cautious, because even if you're 17 and the guy is 21, that's still a significant age gap. When you're under 18, someone who's four years older than you has completely different life goals and priorities than you, and has more life experience than you do (and would be much better at manipulating you or taking advantage of you). While you may or may not be legally in the clear to do so, it isn't a good idea from a developmental standpoint.