Bridesmaid won't wear dress

Her comfort should come before everything. Having her wear a different dress will not really be an issue. Everyone will be looking at you, anyhow. I'd pick your friends comfort over a silly dress. She may have body issues, like a lot of women, and making her take on another role and bow out of your bridesmaids is even more embarrassing. If she is paying for the dress she gets a say in what she wears.

You asked her to stand next to you for a reason, because shes obviously important to you and special in your life.

So I'm confused reading your post, is the odd dress out lower neck/backless/less modest, or are the other three dresses less modest? Just for my clarification.

Honestly, I don't think you should try to make her wear something that she isn't comfortable with. And that's probably an unpopular opinion since most seem to agree that a bridesmaid's only job is to wear the dress and stand up, but, it's really important to me to make sure my friends are comfortable. You mentioned at the beginning of your post that it's important that they like their dresses and feel flattered in them. So stick with that. If they're all in the same color, I don't think one girl wearing a different dress will stand out that much. If I were a bridesmaid and I were told "you can wear any dress in X color", I might be annoyed if I had to change my dress just because the other three bridesmaids have the same taste and I don't.

I feel like the best thing you can do is remove yourself from their dress decisions. You gave them guidelines, and it'll be up to them to follow them. As long as it's in the same color/material, I think it'd be okay. Otherwise, maybe you could see if she'd be willing to wear a shaw or dressy sweater over the top of the less modest dress you want her to wear? Also, you can add illusion straps that cover most of her chest and back. Those are just some options to potentially alter the popular dress to her comfort level. If the cohesive look is what is most important though, then I'd let her back out and hope there are no hard feelings (risky). Beyond that...the only option I see is to get the other girls on board with different dress selections.

Why did you let the girls pick their dress, then not let them do it? If the dress is the same color, length and fabric no one will care that the neckline is different.

Have you asked one of the other three (or all) if they would be willing to wear the dress? If at least one would wear it then it'd be two and two, so no one would look out of place. Or ask everyone to pick a different dress so none are alike. I let my girls pick what they would be comfortable in because I'd never expect them to all like the same or wear it. One of my girls loved a dress online and on the mannequin, but tried it on and hated it. Have they tried on the dress yet?

You can't really tell your girls that they get to select their own dresses, then tell one of them that not only does she not get to pick her own dress, but she also has to wear one that exposes more of her body than she's comfortable with or not be in the wedding. You can say that you don't want to pick a dress over a friend, but that's exactly what you're doing.

You're trying to make her wear something that she has clearly told you she is not comfortable with. That's the definition of prioritizing the dress over the person.

She's not comfortable with exposing as much skin as the other girls. I suppose you could ask her if she would be willing and comfortable enough to wear the dress with a shawl or shrug over it, but she would have to wear the shawl the entire time, not only after the ceremony and pictures. She needs to dress more modestly for the entire day, not only after the ceremony and pictures are over.

If it were me I would just ask the other girls to each pick a different dress in the same color and length and let your friend who wants the modest dress go for it. Their comfort should come before anything!

I asked my girls to each pick a different dress (Azazie, dusk colored, chiffon fabric, and long length were the only requirements), which solved all the problems you're having. Once they chose their dress, they sent it to the group message so nobody chose the same dress. They all picked different styles and looked beautiful! Their comfort was the most important thing.

I know you want the bm's to match but it this really worth essentially ruining a friendship? I would ask the girls to each pick a unique dress. whoever picks first has the most choices, etc. So your 4th bm can get her conservative one, and the other girls probably have lots more options that they can choose from.