My current work around for simple_form and Bootstrap 4 beta to work together (till simple_form allows input fields to have `is-invalid` set on it on error - see https://github.com/plataformatec/simple_form/pull/1476)

application.scss

// Fix for validations on simple form

// Need to import bootstrap functions and mixins if you want to dynamically set colors
// Otherwise replace theme-color... with hex color
@import 'bootstrap/functions';
@import 'bootstrap/variables';
@import 'bootstrap/mixins';

.has-invalid {
  .invalid-feedback {
    display: block;
  }

  label {
    color: theme-color('danger');
  }

  .form-control {
    background: theme-color-level('danger', -10);
    border-color: theme-color('danger');
  }
}

simple_form_bootstrap.rb

# Use this setup block to configure all options available in SimpleForm.
SimpleForm.setup do |config|
  config.error_notification_class = 'alert alert-danger'
  config.button_class = 'btn btn-default'
  config.boolean_label_class = nil

  config.wrappers :vertical_form, tag: 'div', class: 'form-group', error_class: 'has-invalid' do |b|
    b.use :html5
    b.use :placeholder
    b.optional :maxlength
    b.optional :pattern
    b.optional :min_max
    b.optional :readonly
    b.use :label, class: 'control-label'

    b.use :input, class: 'form-control'
    b.use :error, wrap_with: { tag: 'div', class: 'invalid-feedback' }
    b.use :hint,  wrap_with: { tag: 'small', class: 'text-muted' }
  end

  config.wrappers :vertical_file_input, tag: 'div', class: 'form-group', error_class: 'has-invalid' do |b|
    b.use :html5
    b.use :placeholder
    b.optional :maxlength
    b.optional :readonly
    b.use :label, class: 'control-label'

    b.use :input
    b.use :error, wrap_with: { tag: 'div', class: 'invalid-feedback' }
    b.use :hint,  wrap_with: { tag: 'small', class: 'text-muted' }
  end

  config.wrappers :vertical_boolean, tag: 'div', class: 'form-group', error_class: 'has-invalid' do |b|
    b.use :html5
    b.optional :readonly

    b.wrapper tag: 'div', class: 'checkbox' do |ba|
      ba.use :label_input
    end

    b.use :error, wrap_with: { tag: 'div', class: 'invalid-feedback' }
    b.use :hint,  wrap_with: { tag: 'small', class: 'text-muted' }
  end

  config.wrappers :vertical_radio_and_checkboxes, tag: 'div', class: 'form-group', error_class: 'has-invalid' do |b|
    b.use :html5
    b.optional :readonly
    b.use :label, class: 'control-label'

    b.wrapper tag: 'div', class: 'd-flex flex-column-reverse' do |ba|
      ba.use :input
      ba.use :error, wrap_with: { tag: 'div', class: 'invalid-feedback' }
      ba.use :hint,  wrap_with: { tag: 'small', class: 'text-muted' }
    end
  end

  config.wrappers :horizontal_form, tag: 'div', class: 'form-group', error_class: 'has-invalid' do |b|
    b.use :html5
    b.use :placeholder
    b.optional :maxlength
    b.optional :pattern
    b.optional :min_max
    b.optional :readonly
    b.use :label, class: 'col-sm-3 control-label'

    b.wrapper tag: 'div', class: 'col-sm-9' do |ba|
      ba.use :input, class: 'form-control'
      ba.use :error, wrap_with: { tag: 'div', class: 'invalid-feedback' }
      ba.use :hint,  wrap_with: { tag: 'small', class: 'text-muted' }
    end
  end

  config.wrappers :horizontal_file_input, tag: 'div', class: 'form-group', error_class: 'has-invalid' do |b|
    b.use :html5
    b.use :placeholder
    b.optional :maxlength
    b.optional :readonly
    b.use :label, class: 'col-sm-3 control-label'

    b.wrapper tag: 'div', class: 'col-sm-9' do |ba|
      ba.use :input
      ba.use :error, wrap_with: { tag: 'div', class: 'invalid-feedback' }
      ba.use :hint,  wrap_with: { tag: 'small', class: 'text-muted' }
    end
  end

  config.wrappers :horizontal_boolean, tag: 'div', class: 'form-group', error_class: 'has-invalid' do |b|
    b.use :html5
    b.optional :readonly

    b.wrapper tag: 'div', class: 'col-sm-offset-3 col-sm-9' do |wr|
      wr.wrapper tag: 'div', class: 'checkbox' do |ba|
        ba.use :label_input, class: 'col-sm-9'
      end

      wr.use :error, wrap_with: { tag: 'div', class: 'invalid-feedback' }
      wr.use :hint,  wrap_with: { tag: 'small', class: 'text-muted' }
    end
  end

  config.wrappers :horizontal_radio_and_checkboxes, tag: 'div', class: 'form-group', error_class: 'has-invalid' do |b|
    b.use :html5
    b.optional :readonly

    b.use :label, class: 'col-sm-3 control-label'

    b.wrapper tag: 'div', class: 'col-sm-9' do |ba|
      ba.use :input
      ba.use :error, wrap_with: { tag: 'div', class: 'invalid-feedback' }
      ba.use :hint,  wrap_with: { tag: 'small', class: 'text-muted' }
    end
  end

  config.wrappers :inline_form, tag: 'div', class: 'form-group', error_class: 'has-invalid' do |b|
    b.use :html5
    b.use :placeholder
    b.optional :maxlength
    b.optional :pattern
    b.optional :min_max
    b.optional :readonly
    b.use :label, class: 'sr-only'

    b.use :input, class: 'form-control'
    b.use :error, wrap_with: { tag: 'div', class: 'invalid-feedback' }
    b.use :hint,  wrap_with: { tag: 'small', class: 'text-muted' }
  end

  # Wrappers for forms and inputs using the Bootstrap toolkit.
  # Check the Bootstrap docs (http://getbootstrap.com)
  # to learn about the different styles for forms and inputs,
  # buttons and other elements.
  config.default_wrapper = :vertical_form
  config.wrapper_mappings = {
    check_boxes: :vertical_radio_and_checkboxes,
    radio_buttons: :vertical_radio_and_checkboxes,
    file: :vertical_file_input,
    boolean: :vertical_boolean
  }
end