Question

I have to crate a simple CMS using drupal, It has 4 pages and each page has unique design.

Answer 1

I take that the four pages you are talking of are nodes. A theme can have different templates for different content types.
In the same way done by template_preprocess_node(), a module can implement hook_preprocess_node(), and suggest a different template file to use.

// This code is present in template_preprocess_node().
// Clean up name so there are no underscores.
$variables['template_files'][] = 'node-'. $node->type;

// This is what the module custom_module.module can write in custom_module_preprocess_node().
$variables['template_files'][] = 'node-'. $node->uid;

If the custom module wants to be sure that the template it is suggesting is checked before other template files, then it can adopt the following code:

if (isset($variables['template_files']) && is_array($variables['template_files'])) {
  $variables['template_files'] = array_unshift($variables['template_files'], 'node-'. $node->uid);
}

The theme must have a template file with the suggested name (node-1.tpl.php for the node with ID 1, in example), or the default template file node.tpl.php will be used.