The main question would be why you are reindexing nightly, I would guess product import/sync.
To start with, 4-5 hours is disproportionately long for a catalogue of that size. To refresh all the indexes on a store with 250k products/1 store view takes around 25mins. So either you've got a few store views, or a major server configuration issue.
Your proposal simply isn't practical, how would you account for stock discrepancy etc.by running two live DBs simultaneously, then replacing the other.
At this stage, without any detailed information from you, I would start by setting up your server properly for Magento. After that, address why you need to reindex everything, there are probably much more elegant approaches than locking up several tables for hours on end. Disabling FCP and FCC would remove the need to complete those indexes and testing would show if there was any performance hit; we have found that they can be detrimental on certain catalogues. You can also install SOLR to handle the catalog_fulltext_search index, which relieves the burden from MySQL, is faster to reindex and makes the catalogue substantially faster for search and layered navigation.
The right direction would be to consult a professional, to propose a proper solution to suit your needs exactly using the hardware you have available.