How to render an array to twig
Here you declare 

'allofus_four_grids_block' =&gt; [
      'variables' =&gt; [
        'block' =&gt; array('key' =&gt; array()),
      ],
      'path' =&gt; drupal_get_path('theme', 'basis') . '/templates/custom-blocks',
    ],


So you need to use the variable block :

$build = [
      '#theme' =&gt; 'allofus_four_grids_block',
      '#test_var' =&gt; $this-&gt;t('Test Value'),
      '#block' =&gt; array('key' =&gt; $block),
    ];


Not #doubles