Xcode 11.4 beta2 is now available! 
To select this Stack you just have to open your app on bitrise.io,
go to the Workflow tab (Workflow Editor), and on the Stack tab select the Xcode 11.4.x
option and your next build will start on the Xcode 11.4 stack.
Note: you can now select separate stacks for separate workflows!
Read more at: https://blog.bitrise.io/per-workflow-stack-selection-is-here.
You can find the changes/updates here:
https://github.com/bitrise-io/bitrise.io/pull/339/files
If you’d like to add additional tools to be pre-installed you can find the instructions on GitHub, for both the Linux and for the macOS stacks.
Information about Stack types & update schedules can be found here: discuss.bitrise.io/t/281.
Happy Building! 