Better default styling for Related Posts for WP plugin. Replace the default CSS in the Settings → Related Posts → Styling tab.

style.css

.rp4wp-related-posts:after,
.rp4wp-related-posts ul:after {
	clear: both;
	content: "";
	display: table;
}

.rp4wp-related-posts > ul {
	margin-left: 0;
	padding-left: 0;
	list-style: none;
}

.rp4wp-related-posts li {
	clear: both;
	float: left;
	margin-bottom: 1em;
	width: 100%;
}

.rp4wp-related-posts .rp4wp-related-post-image {
	max-width: 25%;
	float: left;
}

.rp4wp-related-posts .rp4wp-related-post-image img {
	max-width: 100%;
}

.rp4wp-related-posts .rp4wp-related-post-content {
	box-sizing: border-box;
	float: left;
	width: 75%;
	padding-left: 5%;
}

@media screen and (min-width:600px) {

	.rp4wp-related-posts ul {
		display: -webkit-box;
		display: -moz-box;
		display: -ms-flexbox;
		display: -webkit-flex;
		display: flex;
	}
	
	.rp4wp-related-posts li {
		padding-right: 3%;
		width: 25%; /* Optionally limit width of each item */
	}

	.rp4wp-related-posts .rp4wp-related-post-image,
	.rp4wp-related-posts .rp4wp-related-post-content {
		max-width: 100%;
		padding-left: 0%;
		width: 100%;
	}
	
}