How do I grab specific posts (by post id) and display the title, featured image, and excerpt?

I have not tested this but based on your question, this should get you on track:

<?php

$id = 4; // The Page or post ID
$page_data = get_post( $id );
$title = $page_data->post_title;
$content = $page_data->post_content;
$excerpt = substr($content, 0, 155);
$featured_image = wp_get_attachment_url( get_post_thumbnail_id($id, 'thumbnail') );

?>

<div class="post">
<h1><?php echo $title; ?></h1>
<div class="featured"><img src="https://wordpress.stackexchange.com/questions/147844/<?php echo $featured_image; ?>" /></div>
<p class="excerpt"><strong><?php echo $excerpt; ?></strong></p>
<p class="content"><?php echo $content; ?></p>
</div>