Lyric discussion by uncas 

Cover art for 4+20 lyrics by Crosby, Stills, Nash & Young

I think it's about a man who sacrifice a lot to become rich: "and he worked like the devil to be more." He makes it: "A different kind of poverty now upsets me so. " but loses his woman on the way and realises that money doesn't make you happy: "I see that it is empty and there's devils in my head"

When he see that he has devils in his head it could be a metaphore for him realising that he has done some wrong things on his way to the top.