We can achieve it through the event and Observer, where will use event sales_quote_address_collect_totals_after.
Need to create a custom module and configure an event. Let’s say our module is Osc_Tax.
Example of events code (app\code\Osc\Tax\etc\events.xml)

Example of Observer code (app\code\Osc\Tax\Observer\Customtax.php)

namespace Osc\Tax\Observer;
 use \Magento\Framework\Event\ObserverInterface;
use \Magento\Framework\Event\Observer;
class Customtax implements ObserverInterface
   public $additionalTax = 4;
    public function execute(Observer $observer)
$getTotal = $observer->getData('total'); 
         if (count($getTotal->getAppliedTaxes()) > 0) { 
            $getTotal->addTotalAmount('tax', $this->additionalTax); 
return $this;     }

In this Example code, $additionalTax variable have additional tax amount.