ejemplo.html

<div class="container">
    <div class="colLeft">
      <div class="item item-1">
        <div class="item item-a">1a</div>
        <div class="item item-b">1b</div>
      </div>
    
      <div class="item item-2">2</div>
    </div>
    <div class="colRight">
      <div class="item item-3">3</div>
      <div class="item item-4">4</div>
    </div>
  </div>
  <style>
    /* BASE STYLES */
  :root {
    --clr-dark: #0f172a;
    --clr-light: #f1f5f9;
    --clr-accent: #e11d48;
  }
  
  *,*::before,*::after {
    box-sizing: border-box;
  }
  
  body {
    margin: 0;
    padding: 0;
    line-height: 1.6;
    word-spacing: 1.4px;
    font-family: "Roboto", sans-serif;
    color: var(--clr-dark);
    background-color: var(--clr-light);
    display: flex;
    justify-content: center;
    align-items: center;
    height: 100vh;
  }
  
  .container {
    width: 50%;
    height: 650px;
    margin: 0 auto;
    border: 10px solid var(--clr-dark);
  }
  
  .item {
    background-color: #fb7185;
    display: flex;
    justify-content: center;
    align-items: center;
    border: 1px solid red;
  }
  
  /* END OF BASE STYLES */
  .container {
    display: grid;
    grid-template-columns: 1fr 1fr;
  }

  @media screen and (max-width: 400px) {
    .container{
        background-color: purple;
        grid-template-columns: 1fr 6fr;
    }
  }
  @media screen and (min-width: 400px) and (max-width: 800px) {
    .container{
        background-color: green;
        grid-template-columns: 1fr 4fr;
    }
  }
  @media screen and (min-width: 800px) and (max-width: 1000px) {
    .container{
        background-color: cyan;
        grid-template-columns: 1fr 2fr;
    }
  }

  .colLeft, .colRight {
    height: 100%;
    border: 5px solid blue;
    
    display: flex;
    flex-flow: column nowrap;
    justify-content: space-evenly;
    align-items: center;
    align-content: space-around;
  }
  .colLeft {
    flex-grow: 1;
    flex-shrink: 10;
  }
  .colRight {
    flex-grow: 1;
    flex-shrink: 1;
  }
  
  .item-1 {
    flex: 0 1 30%;
    width: 95%;
    
    display: flex;
  }
  
  .item-a {
    height: 100%;
    width: 50%;
  }
  
  .item-b {
    height: 100%;
    width: 50%;
  }
  
  .item-2 {
    flex: 0 1 55%;
    width: 95%;
  }
  .item-3 {
    flex: 0 0 55%;
    width: 95%;;
  }
  .item-4 {
    flex: 0 0 30%;
    width: 95%;
  }
</style>