Does size really matter and why?

Im a below average guy 4” and I’m super insecure about it. What’s a good way to mention it before having a long term relationship and her being disappointed after/ breaking up. All time fear is being made fun of for it.I’ve seen a lot of mixed answers on this but not really a good way to bring it up. Like you can’t just tell her on the first date “oh by the way I have a small dick”.