An opening bid or overcall at the four-level is similar to a Three-Bid, but with a longer suit and more playing strength. You should usually have an 8-card or longer suit, and you may have an ace or king outside.
Even at this high level, your bid is still intended as a preempt, so it should be relatively weak in high-card points (no more than 10-11 pts.). With a hand of opening bid strength or better, you can better describe your hand by opening a one-bid and then jumping to game in your suit after partner responds. This allows partner to bid a slam if he has the right type of hand. With a very strong high-card-point hand (21+) and a long suit, open a Strong Two-Bid.
When deciding whether or not to open or overcall with a Four-Bid, consider the vulnerability. If you are not vulnerable, a Four-Bid tends to be weaker and more preemptive. It can be especially effective if the opponents are vulnerable -- the higher you bid, the tougher you make it for them to find their best contract.
Non-vulnerable Four-Bids can be made with hands where you have little hope of making the contract unless partner has a very good hand. A good guideline is to have about 7-8 playing tricks. Some examples of non-vulnerable Four-Bids:
If you're vulnerable, be cautious. A Four-Bid may still be made to preempt the opponents, but you should have more playing strength -- about 8 or 9 tricks in your own hand. You can open a vulnerable Four-Bid with hands like: