If three people no-show, that's different but I still think the single needs to understand, they have no right of way and need to get out of the way of the foursome behind them. One ball.
Yeah it's probably rare, maybe a single paired with a twosome and they bail after 9 etc...At a public course this probably wouldn't happen often but if it does i guess you just need to work it out
At private clubs it's likely a lot more common because the starters aren't going to tell members they can't go out or have to wait for someone. But also, at private clubs members will be able to figure out the situation much easier because people mostly know each other.