There is no reason to preempt (open with a weak 2 or 3) in fourth seat. The purpose of a preempt is to keep opponents out of the bidding or to keep them from finding a contract. If you are in fourth seat and everyone has passed, you know that since neither opponent has an opening hand, the most they can have between them is around 22 High Card Points (HCP) or less and probably don't have a game.
So if you have a weak hand with a good six card suit in 4th seat and you open the bidding, there’s a good likelihood that between them your opponents have more points than you and your partner. If you open with a weak bid and they then enter the bidding you have allowed them to find a part score they could make. As a result, there's no reason to open the bidding with a sub-opening hand in 4th seat. To do so just invites opponents to search for a part score contract they can make. If you don’t have enough HCP to make an opening 1 bid in 4th seat, pass.
As a result of this, a two level opening bid in fourth seat is available for a descriptive bid other than a weak two, if you have an opening hand. I use it to show 12+ HCP with a six card suit. So if I have 12+ HCP with a five card or less suit, I just open at the one level. But if I have a six card suit with an opening hand or better, I open at the two level. This has two positive effects:
1. It more specifically describes your hand to partner and allows you to proceed bidding without having to rebid your suit to show six cards; and
2. It hinders opponents from entering the bidding to find any contract they might have because the level is too high to start exploratory bidding when they know you have an opening hand or better.
With that as a preamble, here's a hand we held recently:
West (Me) East
♠ K3 ♠ 84
♥ KQ972 ♥ Void
♦ J93 ♦ AKQ87542
♣ 986 ♣ AK3
South West North East
P P P! 2D
P 2H P 3C
P 3N P P
My partner played our system and opened 2D, although it's puzzling why North did not preempt with 2 spades in third seat. There is a feeling that you should not preempt if you have an outside four card major. But in this situation, North, in third seat, must preempt with 2 spades, holding six spades and three of the top five honors. He should forget his four little hearts, especially in third seat. If North preempts with 2S, it would make our finding 3NT extremely difficult, if not impossible. But when your opponents' make a mistake, take advantage of it, and we did.
I felt my partner probably had a pretty good hand, 15-16 HCP; I had no reason for that other than instinct. I did know that she had an opening hand with six diamonds, so I immediately thought we had a shot at 3NT since I had three diamonds to an honor. My heart bid showed a good five card suit. Since I already knew she had at least six diamonds, she was free to bid 3 clubs to show a club stopper. That's all I wanted to know. When she had my unstopped suit and I had a spade stopper, I bid 3NT, expecting a Spade lead into my king doubleton.
You can see that if North leads a heart, South takes the ace and if she shifts to the queen of spades through my doubleton king they can defeat the contract, taking one heart and six spades before I can get in.But, as anticipated, I got the opening lead of the jack of spades (the unbid suit and the standard lead given North’s spade holding) and we took 11 tricks (one spade, eight diamonds, and two clubs) off the top.