Magento 1.9 – Add Field in Admin Panel System Configuration General Tab

Assuming you mean the 404 page where there's admin chrome and 404 text in the content area, it sounds like your ACL is setup incorrectly or your current user account lacks the proper ACL credentials to view your new System Configuration section.

First, if your user has the all ACL permissions (i.e. is a system root user and/or the default user created during setup), you'll need to clear your admin session before being able to see the page. Magento caches the individual permissions an admin user has in the session, not the cache. This happens when you login — so your new ACL rule hasn't been added to the list of rules allowed by your user.

Second, it's always best to go right to the source of an error. Take a look at the following section of code.

#File: app/code/core/Mage/Adminhtml/controllers/System/ConfigController.php
protected function _isSectionAllowed($section)
{
    try {
        $session = Mage::getSingleton('admin/session');
        $resourceLookup = "admin/system/config/{$section}";
        if ($session->getData('acl') instanceof Mage_Admin_Model_Acl) {
            $resourceId = $session->getData('acl')->get($resourceLookup)->getResourceId();
            if (!$session->isAllowed($resourceId)) {
                throw new Exception('');
            }
            return true;
        }
    }
    catch (Zend_Acl_Exception $e) {
        $this->norouteAction();
        $this->setFlag('', self::FLAG_NO_DISPATCH, true);
        return false;
    }
    catch (Exception $e) {
        $this->deniedAction();
        $this->setFlag('', self::FLAG_NO_DISPATCH, true);
        return false;
    }
}

This is what triggers a 404 page if you're missing an ACL role. A

var_dump($e->getMessage());
var_dump($resourceLookup);

in each of the catch blocks will tell you why Magento thinks your user doesn't have the right access permissions.

You can specify a frontend model to your field like this:

<field_name translate="label">
    <label>Field name</label>
    <frontend_type>select</frontend_type>
    <frontend_model>module/field_name_renderer</frontend_model><!-- this is it -->
    <source_model>adminhtml/system_config_source_yesno</source_model>
    <sort_order>10</sort_order>
    <show_in_default>1</show_in_default>
    <show_in_website>1</show_in_website>
    <show_in_store>1</show_in_store>
</field_name>

Now you have to create the class for that frontend model. This is not actually a model it's a block.

<?php 
class Namespace_Module_Block_Field_Name_Renderer extends Mage_Adminhtml_Block_System_Config_Form_Field{
    protected function _getElementHtml($element) {
        if (YOUR CONDITION HERE) {
            $element->setDisabled('disabled');
        }
        return parent::_getElementHtml($element);
    }
}

As for side note...I have no idea...yet.