Are you a Mac user? I think .sea files are from Stuffit. Which doesn't seem to be used much any more, though I'm not a Mac user so I'm not sure.

If you bought V4 from DAZ, you should be able re-download the file from your account. Download the zip; it will end in .zip. You might find that easier, at least if you're used to installing stuff into Poser.

You can also use their install manager, though I find it a pain. The install manager files end in .dsx.