You're right...4 years is quite a lot at your age. Although you're mature for a 16 year old, it sounds like your friend isn't as mature as the average 20 year old in certain areas of life due to what she's been through.

Fact is.... online relationships/friendships are different to "real" relationships. Even if you've talked for two years online, you don't really know each other. You build up an image of each other and start having feelings for that image. I don't want to say that your feelings aren't real, but just to point out that your infatuation (love only develops only time when you regularly spend time together) might fade once you get to know each other more closesly.

I just don't know if a relationship like this is going to work out. Hannah is 4 years older than me, she's lived 25% longer than I have. By the time I go to university she will be starting her career.

Well, chances that any relationship you have at this point will last are quite small. Even more so if you're with someone 4 years older. She should be at a completely different life stage, having more experience and a completely different timeline in mind.

You also need to consider the reactions of others. Your friend will be judged hard... your friends might high-five you for being with an older girl, but she won't get as much support.