Updated
A Rule 4 deduction is money taken off your winnings when a horse is withdrawn from a race after you've placed a fixed-odds bet on a different runner. It compensates the bookmaker because a smaller field improves the remaining horses' winning chances. The deduction scales with the withdrawn horse's price — from 5p in the pound at the bottom of the scale to 90p for a strong favourite — and horses withdrawn at roughly 14/1 or bigger trigger no deduction at all. It applies to fixed-odds bookmaker bets, not exchange bets.
A Rule 4 deduction is a reduction bookmakers apply to winning fixed-odds bets when another horse in the same race is withdrawn after you've struck your bet. It exists because pulling a runner shortens the field, which improves the remaining horses' chances and would otherwise hand punters a free edge. The deduction only comes off your winnings, never your stake, and only applies if your selection actually goes on to win or place.
The deduction scales with how strongly fancied the withdrawn horse was. A genuine rank outsider pulled out makes so little difference that no deduction applies at all — under the standard scale, withdrawals at roughly 14/1 or bigger are deduction-free. From there the scale steps up: around 5p in the pound at the bottom, rising to 90p when a very short-priced market leader comes out and reshapes the whole race. Bookmakers publish the full scale and base it on the withdrawn horse's price at the moment it was taken out.
Not by that name. On an exchange like Betfair, bets already matched on the withdrawn horse are voided, but bets matched on the remaining runners before the withdrawal have their odds cut by a 'reduction factor' — the exchange's own equivalent adjustment, applied automatically. So a matched exchange bet isn't immune to a price cut after a non-runner, it's just calculated differently from a bookmaker's Rule 4 scale.
No. Best Odds Guaranteed only protects the price you're paid at, it doesn't exempt a bet from a Rule 4 deduction if a market rival is withdrawn. The two rules work independently: BOG decides which price wins if the SP moves in your favour, Rule 4 decides how much comes off for a race that ran with fewer runners than when you struck your bet.
It only bites if your selection actually wins or is placed and pays out — if the bet loses, there's nothing to take a deduction from. It's also only triggered by a genuine late withdrawal after you've already placed your bet; if a horse was already a non-runner before you backed, the price you took had already adjusted for the smaller field.
Yes — Racing Alpha's Rule 4 calculator lets you enter your stake, the odds you're backing at and the withdrawn horse's price, and it works out the exact deduction and your adjusted return in seconds, so a late withdrawal on the way to the off never comes as a surprise once the race is settled.
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