There is one major difference about opening in fourth seat from all other seats: you know exactly what the final result will be if you pass -- the board will be passed out, for a score of 0 to both sides!
In third seat, if you open light, it is with an expectation that your side was outgunned. You expect to go minus on the board whether you open or not, so you are less afraid of suffering a small penalty if you open a subminimum hand and get stuck declaring. You preempt higher because if you have a very weak hand in third seat, you are almost certain your opponents have a game available.
The situation is just the opposite in fourth seat. It is unwise to open in fourth seat unless it is with the intention of making your contract.
If you have a strong hand -- say more than 14 HCP -- you should certainly make your normal opening bid. Your side is almost guaranteed to have more than half the deck, and you plan to have a constructive auction to a makeable contract - quite possibly a game. You even have a fair chance of not getting serious interference from your opponents, who have neither the strength to open at the 1-level nor the distribution to preempt.
If you hold a minimum opening in fourth seat, neither side is likely to have game -- and both sides are likely to have a partscore. If you open with only 11-13 points in fourth seat, you have to be planning to out-compete the opponents at the 2- or 3-level. This leads us to the single most important factor in deciding whether to open a minimum hand in fourth seat:Do you have the major suits -- especially spades? If your fit is higher-ranking than your opponents' fit, you have an advantage. If theirs is higher ranking, you face an uphill struggle for a plus score. In first seat, x Kxx AQxxx KJxx would be a routine opening; in fourth seat, seriously consider passing that hand, because there is such a risk that the result will be 2 or 2 made against you (or 3 or 3 your way down one.) There are many hands that are perfectly good first-, second-, or third-seat openings that are not worth opening in fourth seat. The only subminimum hands worth opening in fourth seat are ones with a clear advantage in a partscore struggle.
Some people strongly advocate opening 4-card majors in fourth seat just as much as they do in third seat. I do not. In Part II I discussed the safety issues regarding opening ratty 4-card suits in third seat. Those same factors apply in fourth seat: you don't want partner raising one time too many on those partscore hands and negating the advantage of holding the higher-ranking suit because you don't know how many trumps you have.
Most of the advantages of having a 4-card spade suit apply whether you open in spades or not: your partner can easily name spades if he has four of them too, even if LHO overcalls, especially if you play negative doubles. And if the opponents try to bid spades themselves, a suit like KT9x that isn't good enough to open is more than good enough to give your side two stoppers in notrump - or two defensive tricks if the opponents declare in spades and run into a bad break.
One popular rule of thumb for fourth-seat openings is "Pearson's Rule" or the "Rule of 15": open if your HCP total plus your spade length is 15 or more. As bidding rules based on high-card points go, that's one of the better ones out there.
If you do open light in fourth seat (because you like your major-suit holdings), bear in mind the cautions from Part II about responder's new suits not being 100% forcing anymore and the need to be careful not to fool partner into thinking you have a strong hand with your rebid. Here are a few examples:
Calling these preempts is a bit of a misnomer. The classical preempt is a bid that risks a game-sized penalty if partner has nothing (on the theory that if partner has nothing, the opponents should be able to make a game.) But after your opponents have already announced they have no interest in even looking for game, you no longer want to be offering them even 200- and 300-point penalties if you can help it. Unless your think you will make your bid if you catch one or two useful cards in your partner's hand, you should pass and avoid any risk of a minus.
I have two basic requirements for a fourth-seat opening of 2 or higher:
The first rule comes straight from the paragraph above: your partner is a passed hand; you really can't hope for more than two or three nice cards in his hand. And the odds are good that some of partner's strength will be wasted opposite shortness in your hand. Unless you are sure partner can contribute two tricks more than half the time, you can't afford to overbid by two tricks.
The second rule is a practical matter that you don't often see discussed in books. Partner needs to be able to count how many of your loser he can cover with confidence. Make his job easy for him. if you preempt in fourth seat, you are saying to your partner, "count your trump face cards and your side sure winners, and raise me only to a level I can make." Let's look at some examples:
To sum up: In fourth seat, open only when you expect a plus score from bidding on.