Shorter is GENERALLY better, but not ALWAYS.

Consider world-of-lucid-dreaming.com. That's four words! This is a successful SBI site created by an eFriend of mine, and she makes over $2,000 a month from her part-time online efforts.

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I actually like the hyphenated domains. I don't think the average surfer thinks about key word spamming when hey see hyphens. Besides, it is a good way to get the perfect domain for searches. Instead of DallasCowboys.com, you could have Dallas-Cowboys.com and I believe the hyphen is read as a space by the SE's.

I believe long hyphenated domains can be a plus if you are going for strictly SE traffic. If you are going the traditional advertising route (printing fliers, signs), you should purchase a shorter one.

Now, of course this could all change if Google has a problem with it. But, so far they don't.

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I've always been told less is more, but I guess it depends on how catchy your 4 words are. But if your words are short, then it might not be too bad. Good luck.

I think shorter domains are better because they are easier to remember, and they sound better. My domain actually has 5 words but it does okay. I couldn't come up with a 2 or 3 word domain that I liked, and that was still available.

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Personal opinion - feel free to disregard...

If you are going after the search engine traffic:

I think that it doesn't really matter how many words, but that the domain name should include some form of branding in it. Otherwise, who are you compared to all the other similar domains? (If this is the case in your niche)

If you are sending people offline there:

Shorter is better. But you can get around it by having a short domain, could even be an acronym of your actual domain, as a redirect to your site. That way you have the best of both worlds.

All the best!