$ composer create-project --keep-vcs --dev silverstripe/installer ./my_silverstripe 4.0.x-dev
You are using the deprecated option "dev". Dev packages are installed by default now.
Installing silverstripe/installer (4.0.x-dev 63186c87fe3958c1c76ed53c81cfddccd3cc1963)
  - Installing silverstripe/installer (dev-master master): Cloning master from cache
Created project in ./my_silverstripe
Loading composer repositories with package information
Updating dependencies (including require-dev)
Your requirements could not be resolved to an installable set of packages.

  Problem 1
    - don't install silverstripe/framework 4.0.x-dev|install silverstripe/framework dev-master
    - Conclusion: don't install symfony/yaml v3.2.7
    - Conclusion: don't install symfony/yaml v2.8.19
    - Conclusion: don't install symfony/yaml v2.8.18
    - Conclusion: don't install symfony/yaml v3.2.6
    - Conclusion: don't install symfony/yaml v2.8.17
    - Conclusion: don't install symfony/yaml v2.8.16
    - Conclusion: don't install symfony/yaml v3.2.5
    - Conclusion: don't install symfony/yaml v2.8.15
    - Conclusion: don't install symfony/yaml v2.8.14
    - Conclusion: don't install symfony/yaml v3.2.4
    - Conclusion: don't install symfony/yaml v2.8.13
    - Conclusion: don't install symfony/yaml v2.8.12
    - Conclusion: don't install symfony/yaml v3.2.3
    - Conclusion: don't install symfony/yaml v2.8.11
    - Conclusion: don't install symfony/yaml v2.8.10
    - Conclusion: don't install symfony/yaml v3.2.2
    - Conclusion: don't install symfony/yaml v2.8.9
    - Conclusion: don't install symfony/yaml v2.8.8
    - Conclusion: don't install symfony/yaml v3.2.1
    - Conclusion: don't install symfony/yaml v2.8.7
    - Conclusion: don't install symfony/yaml v2.8.6
    - Conclusion: don't install symfony/yaml v3.2.0
    - Conclusion: don't install symfony/yaml v2.8.5
    - Conclusion: don't install symfony/yaml v2.8.4
    - Conclusion: don't install symfony/yaml v3.2.0-RC2
    - Conclusion: don't install symfony/yaml v2.8.3
    - Conclusion: don't install symfony/yaml v2.8.2
    - Conclusion: don't install symfony/yaml v3.2.0-RC1
    - Conclusion: don't install symfony/yaml v2.8.1
    - Conclusion: don't install symfony/yaml v2.8.0
    - Conclusion: don't install symfony/yaml v3.2.0-BETA1
    - Conclusion: don't install symfony/yaml v2.7.26
    - Conclusion: don't install symfony/yaml v2.7.25
    - Installation request for silverstripe/config 1.0.x-dev -> satisfiable by silverstripe/config[1.0.x-dev].
    - Installation request for silverstripe/framework 4.0.x-dev -> satisfiable by silverstripe/framework[4.0.x-dev].
    - silverstripe/framework dev-master requires symfony/yaml ~2.7 -> satisfiable by symfony/yaml[2.7.x-dev, 2.8.x-dev, v2.7.0, v2.7.0-BETA1, v2.7.0-BETA2, v2.7.1, v2.7.10, v2.7.11, v2.7.12, v2.7.13, v2.7.14, v2.7.15, v2.7.16, v2.7.17, v2.7.18, v2.7.19, v2.7.2, v2.7.20, v2.7.21, v2.7.22, v2.7.23, v2.7.24, v2.7.25, v2.7.26, v2.7.3, v2.7.4, v2.7.5, v2.7.6, v2.7.7, v2.7.8, v2.7.9, v2.8.0, v2.8.0-BETA1, v2.8.1, v2.8.10, v2.8.11, v2.8.12, v2.8.13, v2.8.14, v2.8.15, v2.8.16, v2.8.17, v2.8.18, v2.8.19, v2.8.2, v2.8.3, v2.8.4, v2.8.5, v2.8.6, v2.8.7, v2.8.8, v2.8.9].
    - Can only install one of: symfony/yaml[3.2.x-dev, 2.7.x-dev].
    - Can only install one of: symfony/yaml[3.2.x-dev, 2.8.x-dev].
    - Can only install one of: symfony/yaml[v2.7.0, 3.2.x-dev].
    - Can only install one of: symfony/yaml[v2.7.0-BETA1, 3.2.x-dev].
    - Can only install one of: symfony/yaml[v2.7.0-BETA2, 3.2.x-dev].
    - Can only install one of: symfony/yaml[v2.7.1, 3.2.x-dev].
    - Can only install one of: symfony/yaml[v2.7.10, 3.2.x-dev].
    - Can only install one of: symfony/yaml[v2.7.11, 3.2.x-dev].
    - Can only install one of: symfony/yaml[v2.7.12, 3.2.x-dev].
    - Can only install one of: symfony/yaml[v2.7.13, 3.2.x-dev].
    - Can only install one of: symfony/yaml[v2.7.14, 3.2.x-dev].
    - Can only install one of: symfony/yaml[v2.7.15, 3.2.x-dev].
    - Can only install one of: symfony/yaml[v2.7.16, 3.2.x-dev].
    - Can only install one of: symfony/yaml[v2.7.17, 3.2.x-dev].
    - Can only install one of: symfony/yaml[v2.7.18, 3.2.x-dev].
    - Can only install one of: symfony/yaml[v2.7.19, 3.2.x-dev].
    - Can only install one of: symfony/yaml[v2.7.2, 3.2.x-dev].
    - Can only install one of: symfony/yaml[v2.7.20, 3.2.x-dev].
    - Can only install one of: symfony/yaml[v2.7.21, 3.2.x-dev].
    - Can only install one of: symfony/yaml[v2.7.22, 3.2.x-dev].
    - Can only install one of: symfony/yaml[v2.7.23, 3.2.x-dev].
    - Can only install one of: symfony/yaml[v2.7.24, 3.2.x-dev].
    - Can only install one of: symfony/yaml[v2.7.3, 3.2.x-dev].
    - Can only install one of: symfony/yaml[v2.7.4, 3.2.x-dev].
    - Can only install one of: symfony/yaml[v2.7.5, 3.2.x-dev].
    - Can only install one of: symfony/yaml[v2.7.6, 3.2.x-dev].
    - Can only install one of: symfony/yaml[v2.7.7, 3.2.x-dev].
    - Can only install one of: symfony/yaml[v2.7.8, 3.2.x-dev].
    - Can only install one of: symfony/yaml[v2.7.9, 3.2.x-dev].
    - Can only install one of: symfony/yaml[v2.8.0-BETA1, 3.2.x-dev].
    - silverstripe/config 1.0.x-dev requires symfony/yaml ^3.2 -> satisfiable by symfony/yaml[3.2.x-dev, 3.3.x-dev, v3.2.0, v3.2.0-BETA1, v3.2.0-RC1, v3.2.0-RC2, v3.2.1, v3.2.2, v3.2.3, v3.2.4, v3.2.5, v3.2.6, v3.2.7].
    - Conclusion: don't install symfony/yaml 3.3.x-dev