Bootstrap 4
Bootstrap 4 Buttons
Bootstrap 4 provides nine predefined button styles. Use Bootstrap's .btn class, followed by the desired styles .btn-primary, .btn-success, .btn-warning, .btn-secondary, .btn-info, .btn-danger, .btn-light, .btn-dark, .btn-link. For example class="btn btn-primary"  results in a primary button.
<button type="button" class="btn btn-primary">Primary</button>
<button type="button" class="btn btn-success">Success</button>
<button type="button" class="btn btn-warning">Warning</button>
<button type="button" class="btn btn-secondary">Secondary</button>
<button type="button" class="btn btn-info">Info</button>
<button type="button" class="btn btn-danger">Danger</button>
<button type="button" class="btn btn-light">Light</button>
<button type="button" class="btn btn-dark">Dark</button>
<button type="button" class="btn btn-link">Link</button>
Output :
Button tags

The .btn classes are designed to be used with the <button> element. However, you can also use these classes on <a> or <input> elements (though some browsers may apply a slightly different rendering).

<a class="btn btn-primary" href="#" role="button">Submit</a>
<button class="btn btn-primary" type="submit">Sign In</button>
<input class="btn btn-primary" type="button" value="Sign Up">
<input class="btn btn-primary" type="submit" value="Login">
<input class="btn btn-primary" type="reset" value="Register">
Output :
Button Sizes

Bootstrap 4 buttons are .btn-lg, .btn-sm or .btn-block for additional sizes.

<button type="button" class="btn btn-primary btn-lg">Large button</button>
<button type="button" class="btn btn-secondary btn-lg">Large button</button>

<hr />

<button type="button" class="btn btn-primary btn-sm">Small button</button>
<button type="button" class="btn btn-secondary btn-sm">Small button</button>

<hr />

<button type="button" class="btn btn-primary btn-lg btn-block">Block level button</button>
<button type="button" class="btn btn-secondary btn-lg btn-block">Block level button</button>
Output :
Active & Disabled state

The Bootstrap 4 Active State Button are .active (and include the aria-pressed="true" attribute) should you need to replicate the state programmatically and Disables State Button are adding the disabled boolean attribute to any <button> element.

<a href="#" class="btn btn-primary btn-lg active" role="button" aria-pressed="true">Active link</a>
<a href="#" class="btn btn-secondary btn-lg active" role="button" aria-pressed="true">Link</a>

<hr />

<button type="button" class="btn btn-lg btn-primary" disabled>Disabled button</button>
<button type="button" class="btn btn-secondary btn-lg" disabled>Button</button>
Output :
Checkbox and radio buttons

The Bootstrap 4 Active State Button are .active (and include the aria-pressed="true" attribute) should you need to replicate the state programmatically and Disables State Button are adding the disabled boolean attribute to any <button> element.

<h4>Checkboxes</h4>
<div class="btn-group" data-toggle="buttons">
  <label class="btn btn-secondary active">
    <input type="checkbox" checked autocomplete="off"> HTML5
  </label>
  <label class="btn btn-secondary">
    <input type="checkbox" autocomplete="off"> CSS3
  </label>
  <label class="btn btn-secondary">
    <input type="checkbox" autocomplete="off"> Bootstrap 4
  </label>
</div>

<hr />

<h4>Radio buttons</h4>
<div class="btn-group" data-toggle="buttons">
  <label class="btn btn-primary active">
    <input type="radio" name="options" id="" autocomplete="off" checked> PHP
  </label>
  <label class="btn btn-primary">
    <input type="radio" name="options" id="" autocomplete="off"> Python
  </label>
  <label class="btn btn-primary">
    <input type="radio" name="options" id="" autocomplete="off"> Java
  </label>
</div>
Output :