I would believe your wife. Look at this thread.
Post number 71.
"Well I will happily admit to having a smaller guy. I’m a bit, uhhh, abnormally tight so he’s perfect for me. Although even his four inches* can hurt if there’s not enough foreplay and doggy hurts too much to do. If I were with one of these 8+ inch guys I don’t there would be any hope of me ever enjoying sex.
*Complete guesstimation. He’s self-conscious about his size (although needlessly so) so I would NEVER suggest measuring him and putting him on the spot like that"
Women who are satisfied with 4 inches do exist. I really do believe that you got a little lucky that your wife is compatible with your size.