You can set this on each product in the backend in the design tab set :
<reference name="product.info">
<action method="setTemplate"><template>catalog/product/newview.phtml</template></action>
</reference>
Else you can also perform this through an Observer to get all product from category X in one shot.
Create your own module observing the controller_action_layout_generate_blocks_after
event with a function like this one :
public function generateBlocksAfter($event)
{
$controller = $event->getAction();
//limit to the product view page
if($controller->getFullActionName() != 'catalog_product_view')
{
return;
}
$layout = $controller->getLayout();
$product_info = $layout->getBlock('product.info');
if(!$product_info)
{
Mage::log('Could not find product.info block');
return;
}
$id = Mage::registry('current_product')->getId();
$prod = Mage::getModel('catalog/product')->load($id);
$category_ids = $prod->getCategoryIds();
if(in_array(3,$category_ids) || in_array(4,$category_ids))
$product_info->setTemplate('catalog/product/newview.phtml');
}