If you're just too lazy to indent a huge number of lines, you can select it and click on the {}
-button in the panel above the text field where you enter your post.
Another possibility is to use the HTML tag pair <pre><code> ... </code></pre>
for code formatting. It means "preformatted text" and "code" on the finally displayed site it is exactly the same as code blocks indented with 4 spaces, because those Markdown indentations are converted to exactly this HTML tag pair. Note that you should write the tags in the same line as the first (last) line of your code snippet, according to @EliahKagan.
Example (written with indentation to escape the HTML tags and make them visible!):
<pre></code>#<pre><code># Here come some code lines
# ...
exit</code></pre>