I'm in love with my best friend and I have been for the past 4 years
Why don't you want to tell them about it?
I don't want to ruin our friendship and we can be better as friends than as lovers
If you feel that strongly after 4 years, I think that you should tell them. Why hide your true feelings for any longer?
But what if they don't love me back?
What if they love you too? You'll regret it later for not telling them how you feel about them. Maybe they're having the same feelings?