Botones hover (CSS)

<style>
section {
  display: flex;
  position: absolute;
  transform: translate3d(-50%, -50%, 0);
  left: 50%;
  top: 50%;
}

div {
  padding: 20px;
  margin: 5px;
  background: orange;
  border-radius: 7px;
  transition: .3s;
}

section:hover > :not(:hover) {
  opacity: .4;
  background: blue;
  color: white;
}
</style>

<section>
  <div>1</div>
  <div>2</div>
  <div>3</div>
  <div>4</div>
</section>