my previous answer was locked by this bulletin system, so I re-type it here, sorry--
Thanks @Jo , I was used to manually add all classes. And that was fine to me. Then I tried to get used to add them with the panel, but some of them are missing and it's slower for me. But now I don't know what's the best way to operate. I can't do all from the panels, and I find quite wrong to add some classes manually and other classes from the panel. That's an inconsistency.
But I still think the most troublesome issue is not supporting an external editor or import-export feature. So I just use BSS to prototype and then start working with Atom, and all bootstrap4 packages. Adding a class= "small muted pl-2" to a header is much faster with a serious editor that to click in a few different panels, toggle a few switches and type only part of that. Especially on a multi-monitor setup.
(my previous sample was pasted wring: i meant:
<h1 class="display-1">Display-1</h1>
<p>Lead Paragraph</p>
but I'm sure you got the point. some bootstrap4 options are available from the options pane, where you have to flip switches, other need to be manually typed. And there is no easy, simple, quick way to add a class= "small muted pl-2" to a header. )
My suggestion is to completely drop support to bootstrap3 and implement a more streamlined workflow with boostrap4. Right now BSS is fine for first-time users that build just a single website a don't know bootstrap (and its classes), but it's not always useful to maintain real websites after development. Add PHP to a few forms, do whatever work you need outside BSS and you're SOL. Yes, you could re-export, re-rename, re-do anything, but after months, or year you've deployed a website it's an unnecessary burden. Much easier after prototyping or first development to stay away from BSS and just use Atom or your serious editor of choice, with bootstrap4 support.
Right now Martin seems more interested into adding features for one time users that don't know bootstrap and are not interested into learning how it works. But IMHO that is turning this professional app in the perfect tool to 'pirate' just to do a single website without studying. I'm not worried about investing my time to study, there's always a learning curve, I guess most of us bought some bootstrap4 course and read the complete docs, even if already used to bootstrap3. Not for a single site, but as an investment for future years of website developing. And it feels frustrating sometimes to see that most BSS new features are aimed at the occasional users, that maybe even didn't even buy this app.