Xcode 10 beta4 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 10.0.x
option and your next build will start on the Xcode 10 stack.
Note: you can now select separate stacks for separate workflows!
Read more at: https://blog.bitrise.io/per-workflow-stack-selection-is-here.
This new Virtual Machine replaced the previous Xcode 10.0.x configuration,
so if you’ve already selected the Xcode 10.0.x stack for your app, you’re automatically using Xcode 10 beta4 now.
You can find the changes/updates compared to the Xcode 10 beta3 stack here:
https://github.com/bitrise-io/bitrise.io/pull/228/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! 