Q:

Composer create-project symfony/skeleton new_project "4.0"

It installs:
Symfony 4.1.3 (kernel: src, env: dev, debug: true)
How do I get it to install exactly 4.0.*?
composer create-project symfony/skeleton new_project "4.0" Installing symfony/skeleton (v4.0.0)
  - Installing symfony/skeleton (v4.0.0): Loading from cache Created project in aaaa Loading composer repositories with package information Updating dependencies (including require-dev) Package operations: 22 installs, 0 updates, 0 removals
  - Installing symfony/flex (v1.0.89): Loading from cache
  - Installing symfony/polyfill-mbstring (v1.9.0): Loading from cache
  - Installing symfony/console (v4.1.3): Loading from cache
  - Installing symfony/routing (v4.1.3): Loading from cache
  - Installing symfony/polyfill-ctype (v1.9.0): Loading from cache
  - Installing symfony/http-foundation (v4.1.3): Loading from cache
  - Installing symfony/event-dispatcher (v4.1.3): Loading from cache
  - Installing psr/log (1.0.2): Loading from cache
  - Installing symfony/debug (v4.1.3): Loading from cache
  - Installing symfony/http-kernel (v4.1.3): Loading from cache
  - Installing symfony/finder (v4.1.3): Loading from cache
  - Installing symfony/filesystem (v4.1.3): Loading from cache
  - Installing psr/container (1.0.0): Loading from cache
  - Installing symfony/dependency-injection (v4.1.3): Loading from cache
  - Installing symfony/config (v4.1.3): Loading from cache
  - Installing psr/simple-cache (1.0.1): Loading from cache
  - Installing psr/cache (1.0.1): Loading from cache
  - Installing symfony/cache (v4.1.3): Loading from cache
  - Installing symfony/framework-bundle (v4.1.3): Loading from cache
  - Installing symfony/yaml (v4.1.3): Loading from cache
  - Installing symfony/dotenv (v4.1.3): Loading from cache Writing lock file Generating autoload files Symfony operations: 4 recipes (df8c84407bbee5e3e826455e7bbdef65)
  - Configuring symfony/flex (>=1.0): From github.com/symfony/recipes:master
  - Configuring symfony/framework-bundle (>=3.3): From github.com/symfony/recipes:master
  - Configuring symfony/console (>=3.3): From github.com/symfony/recipes:master
  - Configuring symfony/routing (>=4.0): From github.com/symfony/recipes:master 

Executing script cache:clear [OK] 
Executing script assets:install public [OK]

A:

Try it this way:
composer create-project symfony/skeleton new_project 4.0.*

More info in composer documentation - https://getcomposer.org/doc/03-cli.md#create-project
Also, Symfony 4.0.* is no longer maintained. Consider upgrading to 4.1

A:

Just installing as it is:
composer create-project symfony/skeleton new_project 4.0

it will install 4.1.3 and then changing in composer.json symfony/framework-bundle to "~4.0.0" and then composer update 

