ValidateCartBeforeCheckout

events.xml

<?xml version="1.0"?>
<config xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
        xsi:noNamespaceSchemaLocation="urn:magento:framework:Event/etc/events.xsd">
    <event name="controller_action_predispatch_checkout_index_index">
        <observer name="validate_cart" instance="MageVision\Blog32\Observer\ValidateCartObserver" />
    </event>
</config>

ValidateCartObserver.php

<?php
namespace MageVision\Blog32\Observer;

use Magento\Framework\Event\ObserverInterface;
use Magento\Checkout\Model\Cart as CustomerCart;
use Magento\Framework\Message\ManagerInterface;
use Magento\Framework\App\Response\RedirectInterface;
use Magento\Framework\Event\Observer;

class ValidateCartObserver implements ObserverInterface
{
    /**
     * @var ManagerInterface
     */
    protected $messageManager;

    /**
     * @var RedirectInterface
     */
    protected $redirect;

    /**
     * @var Cart
     */
    protected $cart;

    /**
     * @param ManagerInterface $messageManager
     * @param RedirectInterface $redirect
     * @param CustomerCart $cart
     */
    public function __construct(
        ManagerInterface $messageManager,
        RedirectInterface $redirect,
        CustomerCart $cart
    ) {
        $this->messageManager = $messageManager;
        $this->redirect = $redirect;
        $this->cart = $cart;
    }

    /**
     * Validate Cart Before going to checkout
     * - event: controller_action_predispatch_checkout_index_index
     *
     * @param Observer $observer
     * @return void
     */
    public function execute(Observer $observer)
    {
        $quote = $this->cart->getQuote();
        $controller = $observer->getControllerAction();
        $cartItemsQty = $quote->getItemsQty();

        //If cart item qty is lower than 4 then redirect to the cart with a message
        if ($cartItemsQty < 4) {
            $this->messageManager->addNoticeMessage(
                __('Please review your cart!')
            );
            $this->redirect->redirect($controller->getResponse(), 'checkout/cart');
        }
    }
}

Thank you!