So from a blank slate, something like this?
composer create-project flarum/flarum forum/ --stability=dev
cd forum
Change composer.json package versions to "dev-master as 0.1.0-beta.4" (as above)
Update base install and compile javascript
composer update --prefer-source
( cd vendor/flarum/core && ./scripts/compile.sh )
The result has 0.1.0-beta.4. under You are running version which is a good sign. Also had a database upgrade requested for my mysqldump file I was using..
So I think it's now on dev-master now, at least 0.1.0-beta.4
Since this isn't about my setup and about the bug, I attempted to reproduce it but couldn't. Since I'm not 100% familiar with the steps (the gif moves really fast) maybe someone else could try.