The "Weak 4's" are agressive bids that commit the partnership to taking 10 tricks in the opened suit, without knowing the contents of the responder's hand. This 4-level opening is referred to as a defensive bid - it prevents the opposition from bidding.
These are hands with less than 12 HCPs and 8+cards in the named suit. Weak 4's are part of SAYC, although not all social players use them. It is best to clarify this with the partner before the bridge session begins.
The 4 ♥ and 4 ♠ openings commit the partnership to a game level contract. The responder is expected to PASS.
The 4 ♣ and 4 ♦ openings commit the partnership to one trick shy of a game level contract in the minor suit.
Some players want to open an 8-card minor suit at the 4-level. I suggest NOT opening a minor at the 4-level. This eliminates the possibility of playing the contract at 3 NT, which might be the preferred contract.
Also, the 4-Club opening might be mistaken as a Gerber request for the number of Aces in the partner's hand. Instead, open the 8+card club suit at the 3-level.
The opener should have at least 8 tricks in the hand. The opener hopes the responder will make up the difference by taking at least 2 tricks.
There are 4 requirements:
1) Point Range: Opener must have 9 to 11 HCPs in the hand. 2) Suit Length: Opener must have at least 8+cards in the named suit.
3) Suit Strength (Critical):The opened suit should have at least 5+ HCPs. Many players forget the honors in the named suit, and end up losing the contract.
4) Suit Selection: Open with an 8+card MAJOR suit only.
When one player holds an 8+card suit, do not be surprised that at least one other player will also have a long suit (accompanied by short suits and voids). A key to success is to open the 4-level bid before the opposition has a chance to bid. The opening 4-level shuts-out the opponents bidding.
The responder is expected to PASS the opening 4-level bid.