Hi,
When you save the configuration of your MIGS VPC payment method, it creates a file migsvpc_return.php at the root of your website so that it can handle the return URLs.
If you get a 404 error, it means that HikaShop couldn't write that file. So make sure that the root folder of your website is writable and then save again the settings of your MIGS VPC payment method and it will create that file.
Once the file is there, it should work fine.
You can leave the "return URL" parameter empty. It will redirect to the generic thank you page of HikaShop if you do.