Hm, I wouldn't say it is irrelevant. It would be (more or less) irrelevant if you had OS block size of 4K and DB_BLOCK_SIZE of 8K, but not the other way arround as the original poster asked.

If you have 8K OS block size and 4K database block size, then for every single block I/O operation the database requests, the OS will read an extra 4K of data the database hasn't asked for.