4th Chair Openings:
Rule Of 15

When opening in 4th chair (instead of passing out the hand), you must either have a sound opening hand or, with a light opener, you must have control of the boss suit -- spades. Otherwise, you may open 1 with something like xx-KQxxx-AQJx-xx, only to have the opponents steal the contract in Spades.

The Rule Of 15 says that to open in 4th seat, your total number of Spades added to your HCPs must be at least 15. So you would open 1 with KQxxx-xx-KQxx-xx, but you would not open 1 with xx-KQxxx-AQJx-xx, even though the latter hand has more HCPs.

Max Hardy, Pro:

In Standard Bridge Bidding For The 21st Century, Hardy says that you also should still have 2 defensive ("quick") tricks, just as in 1st seat, but the first example hand he shows is KJ965-AJ3-Q94-63, which does not have 2 quick tricks. We're going to do as he does, not as he says, and open this hand despite the lack of 2 quick tricks.

Max Hardy, Con:

Another interesting hand from the book is KJ96-AJ3-63-Q954. Hardy says to open 1 rather than 1 because that brings Hearts into play.

We have two problems with Hardy's advice. One is that with a weak hand like this, you have to pass any bid by responder, including 1. Hardy mentions this, but doesn't act like it is a problem to possibly end in a 5-2 fit.

An even bigger problem with this hand is that although the Spade suit passes the Rule Of 15, the Heart suit does not (11 HCPs + 3 Hearts), so if you have a Club fit but partner does not have Hearts, the opponents may steal the bid in Hearts, as well as still having a chance for Spades.

BidSim Speaks

To analyze what happens with various light opening bids in 4th seat, we ran the following simulations with BidSim:

KJ965-AT3-Q94-63
LHO and RHO each pass about 95% of the time. This is largely because of having to bid on the 2 level, which normally promises a hand close to opening strength, which the opponents have already denied. It may be that another set of bids is needed for overcalling a 4th-seat opening.

Also notice that 4th seat has 2 of the top 5 cards in Hearts, reducing the chances of the opponents having a suit good enough to overcall with.

We ran SimBid for the hand above and got the following results: 5% chance of opponents bidding Spades, about a 10% chance for Hearts, about 67% for passing, and about 18% for other bids.

As for partner, there's about a 16% chance of his bidding 1, 22% for 1, 25% for 1, but that 25% 1 breaks down into 18% with a 5-2 fit and only 7% with a 6-2 or better fit. Pard will raise Clubs about 5% of the time.

The bottom line is that partner will pass about 17% of the time and the opponents will pass 65% of the time or more. So it appears that it pays to open in 4th seat, even with KJ96-AJ3-63-Q954.

Jump Straight To Game

Another 4th seat opening tactic is to jump to game with a reasonable chance of it, since opposite a passed hand, there is little chance of slam, e.g.: KQJT432-2-A32-K2.

Don't Open a Weak Two

In Feb. 2022 Bridge Bulletin, p. 67, Billy Miller says

It makes little sense that your normal Weak Two bid range should be used in fourth seat. [Because a 6-10 HCP range makes it more likely that the opponents have more points.]