Around 4 inch is a good enough tool. You have a 6 inch boner and want to increase, there is no way right now as your growth time has passed. Still can go for cosmetic surgery if you think size is the only thing a girl cares about.
To satisfy a woman, its never the length of a dick that matters, only male has all the inferiority complex on size matters, the reaseach says woman doesnot care how long a man is, all they care about is who is the man. I saw a TV programme recently, that gave some very good insight, if a girl is in love, just hearing the name gives her a great pleasure. The experiment was done on masturbation also, if she does alone, she reachs an orgasm, if her man's picture even is showed, she reaches a better orgasm.
So many other experiments were done, if I start mentioning, that wont reply your question, but the bottom line remained same everytime, ist the person a girl/woman think about when having sex, its not the tool. But sure some basic rule has to e performed.
As you have mentioned about satisfaction, try to understand the real fact, most male can give a better satisfaction during foreplay, the penetration part is a bonus. Your tool is just a tool, your skill is the art, you yourself is the center of a girl's sexual satisfaction. If she cares for you, everytime she will have a great sex.