I don't assume anything is 4x unless it says it on the packaging--regardless of part number, etc. If it promises 4x on the packaging, you've got a leg to stand on. Much less so if all you can complain about is, "Dude--but it said on the Internet!"
Just last night, I bought some Verbatims at Office Depot without a speed printed on the packaging. The Office Depot guy promised they were 4x, and that he'd take 'em back if they weren't.
As it turns out, they're 2.4x, and are going back this weekend!