Magento 1.8.1 - block type="catalog/product_list" not displaying on the product page

Question

I'm having a problem with the product page on a Magento site I'm working on.

I'm using the following code in a static block to display products in the header section:

{{block type="catalog/product_list" category_id="4" template="catalog/product/custom_list.phtml"}}  

The thing is, the block is displaying fine on all pages except on the "product" page. Am I missing something here?

Hope you guys can help me.

custom_list.phtml:

<?php
    $_productCollection = $this->getLoadedProductCollection();
    $_helper = $this->helper('catalog/output');
?>
<?php if(!$_productCollection->count()): ?>
    <p class="note-msg"><?php echo $this->__('There are no products matching the selection.') ?></p>
<?php else: ?>
    <div class="cat-product-list">
        <ul class="products-grid">
            <?php foreach ($_productCollection as $_product): ?>
                <li class="item">
                    <a href="<?php echo $_product->getProductUrl() ?>" title="<?php echo $this->stripTags($this->getImageLabel($_product, 'small_image'), null, true) ?>" class="product-image"><img src="<?php echo $this->helper('catalog/image')->init($_product, 'small_image')->resize(90,60)->constrainOnly(TRUE)->keepAspectRatio(TRUE)->keepFrame(TRUE); ?>" width="90" height="60" alt="<?php echo $this->stripTags($this->getImageLabel($_product, 'small_image'), null, true) ?>" /></a>
                </li>
            <?php endforeach ?>
        </ul>
    </div>
<?php endif; ?>

Answer 1

It turns out that the products won't show up on the product page unless the category (category_id="4") is the same as the category of the product you are viewing. Found that in List.php.

if (Mage::registry('product')) {
                $categories = Mage::registry('product')->getCategoryCollection()
                    ->setPage(1, 1)
                    ->load();
                if ($categories->count()) {
                    $this->setCategoryId(current($categories->getIterator()));
                }
            }

I've added a local copy of the List.php file and modified it to my needs. It is working fine now.