<div class='well clearfix'>
  <% param_prefix = "payment_source[#{payment_method.id}]" %>
  <p class='field'>
    <%= label_tag :po_number do %>
      <%= Spree.t(:po_number) %><abbr class='required' title='required'>*</abbr>
    <% end %>
    <%= text_field_tag "#{param_prefix}[po_number]", '', id: :po_number, class: 'form-control required', size: 10, maxlength: 10, autocomplete: "off" %>
  </p>
  <p class='field'>
    <%= label_tag :organization_name do %>
      <%= Spree.t(:organization_name) %><abbr class='required' title='required'>*</abbr>
    <% end %>
    <%= text_field_tag "#{param_prefix}[organization_name]", '', id: :organization_name, class: 'form-control required'%>
  </p>
</div>

<div class="payment-gateway">
  <% param_prefix = "payment_source[#{payment_method.id}]" %>

  <div class="payment-gateway-fields">
    <div class="mb-4 payment-gateway-field checkout-content-inner-field">
      <%= text_field_tag "#{param_prefix}[po_number]", '', id: 'po_number', class: 'spree-flat-input', placeholder: Spree.t(:po_number) %>
    </div>

    <div class="payment-gateway-field checkout-content-inner-field">
      <%= text_field_tag "#{param_prefix}[organization_name]", '', id: 'organization_name', class: 'spree-flat-input', placeholder: Spree.t(:organization_name) %>
    </div>
  </div>
</div>

Make template use Spree 4 styles
Make template use Spree 4 styles
