Four or Five Card Majors
Many players claim big advantages from playing 4 or 5 card major openings though there is not actually a great amount of difference. Those playing four card majors generally open the lower of 2 four card suits so the chances are very strong that when they open 1♠ it will be a 5 card suit. The only exception is with exactly 4333 shape and 15-19 hcp. Indeed, if the spade suit is mediocre or worse, there is a good case for opening 1♣ with a 3 card suit as, especially in competitive bidding, you do not want to overstate your spade strength or length. So, a 4 card 1♠ opening is quite rare, even if you are playing 4 card majors.
1♥ is only 4 card when you are 3433 shape (and again 1♣ might be a better opening for reasons stated above) or 44 in the majors with 15- 19 hcp. The discussion about whether to play 4 or 5 card majors revolves largely around a hand with 4-4 in the majors where 4 card advocates do open 1♥. The other difference is that 5 card major advocates have to open 1♣ and maybe 1♦ with less than 4 cards in the suit, which can complicate bidding.
The Advantages of 4 Card Majors
Finding an immediate fit and played by the stronger hand
When you open a 4 card major and your partner has at least four card support, you have found your fit immediately. You do not give out any more information about your hand, which will be the declaring hand. As opener, you are likely (but not guaranteed) to have the higher number of your side’s high card points. This may be significant in the opening lead and the play.
A minor suit opening is always 4+ cards
Although we downplay the worth of finding a club fit and playing in clubs, there are hands where identifying a minor suit fit is very worthwhile. Examples are where 6 of a minor makes 12 tricks whereas 6NT fails by a trick or in low level competitive auctions. There is often a doubt about the length of opener's minor when their partner comes to support the suit. Take a look at the following examples:
Playing 4 card majors, and hence 4 card minors too, South can make a sensible value raise after East’s double. East-West have a choice of defending the cold 3♣ contract or bidding on to 3♥, and be in danger of being 2 down for -200.
Contrast that with:
North's 1♣ opening promised at least 2 clubs. With the same hand as in the first example, South's 3♣ bid is booked to be 2 down. South does not know which of the two North hands they are facing.
While the above are advantages, as stated earlier, we do open 1♣ with 3 clubs which does reduce this advantage of playing 4 card suit openings.
The Advantages of 5 Card Majors
The Raise to 2 of a major
There is a big advantage in bidding as quickly as you can to the level you want to play. When your partner opens 1 of a major, promising 5 cards, the best thing you can do with 6-9 hcp and 3 card support is to raise that major to the 2 level. The opposition have to guess what to do very quickly and are not always correct.
North and South can make 8 tricks in spades but 3♥ should be two down and could be doubled. East and West's club fit got lost. Contrast what could happen playing 4 card majors:
North and South won the contract but West escaped bidding hearts at the 3 level.
The Law of Total Tricks
Many pairs subscribe to the Law of Total Tricks which says you are safe bidding to the level of your trump fit. This means that with a 9 card fit, you should bid to the 3 level as quickly as possible, no matter how few high card points you hold. Thus, holding three points but having 4 trumps, South should bid to 3♠ after their partner opens 1♠. West may have a big problem over 3♠.
The above bidding, where 1♥ - 3♥ (and similarly 1♠-3♠) shows 0-5 hcp is part of the Bergen Raises structure. Although it can be played where opener may have a 4 card major, it is more effective and less dangerous when the opener has at least a five card suit.
Similarly, if partner opens 1♠ and the next hand passes, doubles or even bids a minor, you tend to raise directly to 4♠ with a hand like:
You're not expecting to make your contract but pre-empting your opponents. The bid may not be so successful if both opponents have two spades because your partner has only four. Five card majors are better for such bidding.
Safety in Numbers of Trumps
If responder is too weak to respond, opener is better to have a 5 card opener than 4. Hence, if North opens 1♠ and ends up playing there, having at least 5 trumps in their hand is better than 4. Imagine if your partner had a singleton and was too weak to bid.
Who holds the club suit?
Opening 1♣ with less than 4 cards can make it more difficult for the opponents to identify their own club fit. There can often be doubt as to whether a subsequent club bid by an opposition player does show a genuine club suit.
If you like aggressive bidding with minimal point- count openings, then five card majors are a good idea. A swift 1♠ - 2♠ sequence can often be a winner. If you like certainty in your minor suit holdings, then 4 card majors are for you. You will always find your major fit playing four card majors but it might take an extra round of bidding if the opener’s major is 4 carded.
A final tip. If you are playing 5 card majors, as responder, you might need to bid at the one level with fewer than 6 hcp if you are really short in opener's minor:
South may not relish playing in 3♠ but it is a lot better contract than 1♣ had South passed initially. Beware of possibly disastrous minor fits!