If my partner opens one heart, and I respond one spade, am I showing four or five spades?
In Standard American bidding (and 2/1 game forcing for that matter), after partner's 1H opening, 1S promises at least 4 spades. After 1H-1S, partner's mostly likely calls are 1NT (5332 shape with no more than 14 HCP), 2 of a minor (5-4 or better in the bid suits, may have anything short of a game force in terms of values), 2H (6+ hearts, probably less than 17 high card points), 2NT (18-19 HCP, 5332 shape), or some number of spades.
After 1H-1S-1NT, your rebids:
2H: This is a tough one. Probably 5 spades, a doubleton heart honor, and shortness in one of the minors. You're taking the position that playing 2H in a 5-2 fit is better than playing 1NT (if you had 3 hearts and a weak hand, you shouldn't have bothered to show spades).
2S: The 2S rebid shows a weak hand, usually with 6+ spades (though it could be an exceptionally weak hand with shortness somewhere and only 5 spades, e.g. Qxxxx x Kxx xxxx, which rates to take 0-1 tricks at NT but a few tricks with spades as trumps).
2NT: Invitational values, likely exactly 4 spades and 2 hearts.
3NT: Game forcing values, likely exactly 4 spades and 2 hearts.
2m: You should have a discussion with your partners about this auction, but SAYC says this should be non-forcing and natural.
3H: 3-card heart support, invitational values.
pass: any weakish hand not covered above.
Pass: A weak hand with 3- or 4-card support for the minor and usually a singleton or void in hearts.
2H: A weak hand, virtually guaranteed to have a heart doubleton (again, with 3-card support, why not raise hearts in the first place?)
2S: Shows 6 spades or better (or, more rarely, a weak 5-5 in spades and the unbid minor with a singleton heart). With other shapes and a weak hand, correct to 2H with a doubleton heart or pass partner's minor.
2NT: Balanced, invitational.
3m: 4-card support for the minor, invitational values.
3S: 6+ spades, invitational values
The fourth suit at the cheapest level: forcing for one round, unable to make a natural bid at this point. Examples:
- Interested in 3NT but unable to stop the unbid minor.
- Interested in slam in one of partner's suits and thus unwilling to jump directly to game.
- Interest in a spade slam but require more information from partner
After 1H-1S-2H: - 2S: This shows a weak hand with 6 spades or better and likely a heart void; why else correct from partner's 6-card suit to yours?
2NT: Natural, invitational.
3m: Natural and game forcing.
3H: Invitational with 3 hearts.
Obviously, this doesn't cover all the bases, but it lays out some of the most common situations.
You need only four spades to respond one spade.
When your partner opens, you are expected to try to keep the bidding open if possible. That's why you need only four of a major to respond, and as little six points (although you may have a lot more). The fact that you have four spades is valuable information. Your bid (in a new suit) is forcing, meaning that you will get a chance to bid again and show your other values.
A small percentage of the time, partner will be 5-4 in hearts and spades, and be able to raise spades, and you'll have a 4-4 fit.
More often, there will be a try for a no trump contract. Partner may bid no trump if spades is the only suit where s/he is short. If partner is 5-4 in hearts and a minor, the minor will be bid, and if you have the other minor, you can bid no trump.
Occasionally, you'll find a 4-4 fit in a minor, and make a part score. Game is unlikely unless you and partner have at least 28-29 points between you.