How to add a custom page with cross-sell products

Install below extension that will help you to create extra page after cart

https://github.com/tmhub/suggestpage

above extension add extra page after cart and that page you will add these code and you will get cross-sell product data

change your indexController indexAction as below

public function indexAction()
{
    $this->loadLayout()
        ->_initLayoutMessages('checkout/session')
        ->_initLayoutMessages('catalog/session');

    $_product = Mage::getModel('catalog/product')->load(1); //here 1 is product Id
    $block =  $this->getLayout()->createBlock('checkout/cart_crosssell')->setProduct($_product)->setTemplate('checkout/cart/crosssell.phtml');
    $this->getLayout()->getBlock('content')->append($block);

    $this->renderLayout();
}

and for your 4 product issue please check in your app/design/frontend/YourPackge/YourTheme/checkout/cart/crosssell.phtml file any condition who breaks for loop after four product then change it.

to increase limit for cross-sell product follow this quick step

1) copy app/code/core/Mage/Checkout/Block/Cart/Crosssell.php file to app/code/local/Mage/Checkout/Block/Cart/Crosssell.php

2) find this line and increase limit as per your need protected $_maxItemCount = 4;

We've implemented something on a client website where you click on proceed to checkout you land on a page before it asking you to add free samples, these are pulled in from a category but would be just as easy to pull in some cross-sell/related products.

Essentially code wise what would have to happen is, a module would have to listen for the event controller_action_predispatch_checkout_onepage_index and redirect the user to a page built by that module with the products you require, then after that set a session flag to say this user has already seen this up-sell page and allow them through to the checkout.

I'm not sure if there are any modules out there that currently do this.

As a brief example we have this as our xml:

<controller_action_predispatch_checkout_onepage_index>
    <observers>
        <namespace_module>
            <class>namespace_module/observer</class>
            <method>redirectToUpsellOnCheckout</method>
        </namespace_module>
    </observers>
</controller_action_predispatch_checkout_onepage_index>

Edit 15/11/16 Here's what i have so far, still needs some amends:

app/etc/modules/

<?xml version="1.0"?>
<config>
    <modules>
        <Dc_Crosssells>
            <active>true</active>
            <codePool>community</codePool>
        </Dc_Crosssells>
    </modules>
</config>

app/code/community/Dc/Crosssells/etc/config.xml

<?xml version="1.0"?>
<config>
    <modules>
        <Dc_Crosssells>
            <version>0.0.1</version>
        </Dc_Crosssells>
    </modules>
    <global>
        <blocks>
            <dc_crosssells>
                <class>Dc_Crosssells_Block</class>
            </dc_crosssells>
        </blocks>
        <events>
            <controller_action_predispatch_checkout_onepage_index>
                <observers>
                    <dc_crosssells>
                        <class>Dc_Crosssells_Model_Observer</class>
                        <method>controllerActionPredispatchCheckoutOnepageIndex</method>
                    </dc_crosssells>
                </observers>
            </controller_action_predispatch_checkout_onepage_index>
        </events>
    </global>
    <frontend>
        <routers>
            <beforeyoucheckout>
                <use>standard</use>
                <args>
                    <module>Dc_Crosssells</module>
                    <frontName>before-you-checkout</frontName>
                </args>
            </beforeyoucheckout>
        </routers>
        <layout>
            <updates>
                <dc_crosssells>
                    <file>dc/dc_crosssells.xml</file>
                </dc_crosssells>
            </updates>
        </layout>
    </frontend>
</config>

app/code/community/Dc/Crosssells/Model/Observer.php

<?php
class Dc_Crosssells_Model_Observer
{
    public function controllerActionPredispatchCheckoutOnepageIndex(Varien_Event_Observer $observer)
    {
        if(Mage::getSingleton('core/session')->getCrossSellFlag()) {
            Mage::app()->getResponse()->setRedirect(Mage::getBaseUrl() . 'before-you-checkout');
            Mage::getSingleton('core/session')->setCrossSellFlag('true'); 
        }
    }

}

That above is enough to redirect the user to another page after hitting going to checkout. Would need to set a flag within this observer to the session and then check if that flag exists, if it does, do the redirect.

Then we can create a controller this will be our "before-you-checkout" page, it creates/renders the layout using a block from the template. app/code/community/Dc/Crosssells/controllers/IndexController.php

<?php
class Dc_Crosssells_IndexController extends Mage_Core_Controller_Front_Action
{

    public function indexAction()
    {
        $this->loadLayout(array('default'));

        $block = $this->getLayout()->getBlock('before.you.buy')->getBlockHtml('before.you.buy');
        $this->getLayout()->getBlock('content')->append($block);
        $this->_initLayoutMessages('core/session');
        $this->renderLayout();
    }

}

We have a block class to house the functions needed by the frontend block, I haven't fully tested this, it is supposed to return a product collection filtered by the category you should have created in the backend, in my case it's category 292. app/code/community/Dc/Crosssells/Block/BeforeYouBuy.php

<?php
class Dc_Crosssells_Block_BeforeYouBuy extends Mage_Core_Block_Template
{
    public function getCrossSellProductsFromCategory()
    {
        $category = Mage::getModel('catalog/category')->load(292);

        $products = Mage::getModel('catalog/product')->getCollection()
            ->addCategoryFilter($category)
            ->addAttributeToSelect('*');

        return $products;
    }
}

app/design/frontend/base/default/layout/dc/dc_crosssell.xml

<?xml version="1.0"?>
<layout version="0.1.0">
    <beforeyoucheckout_index_index>
        <reference name="content">
            <block type="dc_crosssells/beforeYouBuy" name="before.you.buy" template="dc/crosssell.phtml"/>
        </reference>
    </beforeyoucheckout_index_index>
</layout>

app/design/frontend/base/default/template/dc/crosssell.phtml

<?php $products = $this->getCrossSellProductsFromCategory();

foreach($products as $item) {
    echo $item->getName(). '<br />';
}

I haven't fully tested this code properly yet and still needs some work to actually display the products properly with add to cart buttons etc. I've put it in a github repo here: https://github.com/DanCarlyon/MagentoCrossSellPage

Feel free to download and have a play with it.