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A rule 4 is a statutory rule of horse racing that reduces the price of a selected horse after the withdrawal of another horse in the race. The rule 4 is there to protect bookmakers from customers gaining an unfair advantage from late withdrawals. If a punter’s winnings were not penalised, it could potentially leave a bookmaker vulnerable to having a negative book. The rule doesn’t apply to bets placed after a horse has been withdrawn. It’s in a bookmaker’s interest to reform a market quickly in order to get punters betting on the race again and non-runners won’t affect the price if a bet is placed once the market is reformed.
When a horse is withdrawn from a race, all the remaining horses have a better chance of winning because they have one fewer horse to beat. The rule 4 fluctuates depending on the price of the horse withdrawn. If an even money shot is withdrawn immediately prior to the race after a punter has bet on another horse, bookmakers will take a 45 pence rule 4. This means that for every £1 you have staked, 45 pence will be taken away. The same rules apply with regards to the betting exchange.
Those unfamiliar with racing may feel this is unfair but it can actually work slightly to a punter’s advantage. If we use an even money shot as an example again, a horse with odds of even money (1/1) would hold a 50% chance of winning the race on a “fair book”. The horse they’ve selected now has a 50% better percentage chance of winning the race but they’re only deducted 45p in their pound. Of course, the punter may have fancied his chances against the even money shot or may have wanted a particular horse in the race with regards to a pace angle but from a statistical point of view the customer is no worse off.
If a horse has greater odds than 14/1 then there will be no rule 4 applied due to them having a less than 5% chance of winning the race. If more than one horse is withdrawn after the customer strikes their bet, then multiple rule 4s can apply. The exception to the rule 4 is ante post betting because the horses aren’t counted as runners at this point. If a horse is withdrawn from an ante post market, bookmakers are not allowed to stipulate a rule 4.