Bridesmaid dilemma

Hi. When I got engaged I asked three of my closest friends to be my bridesmaids. Since then one of my bridesmaids has unfortunately had to pull out and I have became a lot closer with someone else and I would love to ask her to be one of my bridesmaids too. However I dont want her to feel like I'm only asking her to fill the space of my others BM. Does anyone have any ideas of how I can ask her and what I can say to show her that it isn't just to fill the space. Thanks