Fix 404 Error on Startup Page When Changing Admin Panel URL in Magento 1.8

404 issues are quite common when migrating Magento installations between servers. As per this post on SO please see the following answer

Magento Admin 404

You need to go your server directly and do this via SSH/ FTP

You have to delete the following file:

app/etc/use_cache.ser

If you get an error after that like:

Notice: Undefined index: 0 in /srv/www/vhosts/javra.com/htdocs/munchad2/app/code/core/Mage/Core/Model/Mysql4/Config.php on line 92

Then go to your Database Management:

• Open PhpMyAdmin
• Go to your database
• Click SQL
• Run the following SQL Query

_

SET FOREIGN_KEY_CHECKS=0;
UPDATE `core_store` SET store_id = 0 WHERE code='admin';
UPDATE `core_store_group` SET group_id = 0 WHERE name='Default';
UPDATE `core_website` SET website_id = 0 WHERE code='admin';
UPDATE `customer_group` SET customer_group_id = 0 WHERE customer_group_code='NOT LOGGED IN';
SET FOREIGN_KEY_CHECKS=1;

Definitely backup the file & your database prior to running these changes.

Alternately, double check your .htaccess files which are also sometimes the bane of the issue.

The reason for this error is that store_id and website_id for admin should be set to 0 (zero). But, when you import database to new server, somehow these values are not set to 0.

Open PhpMyAdmin and run the following query in your database:-

SET FOREIGN_KEY_CHECKS=0;
UPDATE `core_store` SET store_id = 0 WHERE code='admin';
UPDATE `core_store_group` SET group_id = 0 WHERE name='Default';
UPDATE `core_website` SET website_id = 0 WHERE code='admin';
UPDATE `customer_group` SET customer_group_id = 0 WHERE customer_group_code='NOT LOGGED IN';
SET FOREIGN_KEY_CHECKS=1;

Back up database before making any changes also see log file to find exact issue. Also you can refer link 404 magento admin error Hope this is helpful to you