Rule 4 is an industry-standard deduction that is made on a horse or dog when there is a non-runner in a race after the final declarations for that race have been made.
If you bet on a horse at odds of 4/1 (5.0 in decimal odds) and the 2/1 (3.0) favourite then becomes a non-runner, your odds are now very generous so a deduction is made to be fair to the bookies.
Rule 4 deductions only occur after the final declarations for a race are made. This is when non-runners mean you get your stake back. The final declaration stage is usually 24 hours before the race, although it can be 48 hours before a race.
Rule 4 does not apply to any ante-post market. Ante-post or futures markets are betting on a horse race before the final declarations are known. If you back a non-runner in an ante-post market then you do not get your money back so no Rule 4 is applied to the people who are have placed bets on horses that benefit from this non-runner.
The official Tattersalls Rule 4 deductions, as applied by all UK bookies, are as follows:
a) If the current odds of the non-runner are 1/9 or shorter at the time the non-runner withdraws from the race, then 90p in £/E/$ is deducted (or 90% of winnings)
b) If over 2/11 up to and including 2/17, 85% of winnings deducted
c) If over 1/4 up to and including 1/5, 80% of winnings deducted
d) If over 3/10 up to & including 2/5, 70% of winnings deducted
e) If over 2/5 up to and including 8/15, 65% of winnings deducted
f) If over 8/15 up to and including 8/13, 60% of winnings deducted
g) If over 8/13 up to and including 4/5, 55% of winnings deducted
h) If over 4/5 up to and including 20/21, 50% of winnings deducted
i) If over 20/21 up to and including 6/5, 45% of winnings deducted
j) If over 6/5 up to and including 6/4, 40% of winnings deducted
k) If over 6/4 up to and including 7/4, 35% of winnings deducted
l) If over 7/4 up to and including 9/4, 30% of winnings deducted
m) If over 9/4 up to and including 3/1, 25% of winnings deducted
n) If over 3/1 up to and including 4/1, 20% of winnings deducted
o) If over 4/1 up to and including 11/2, 15% of winnings deducted
p) If over 11/2 up to and including 9/1, 10% of winnings deducted
q) If over 9/1 up to and including 14/1, 5% of winnings deducted
r) If the non-runner is over 14/1 then there is no deduction
In the event of there being two or more withdrawals in one event, the total deduction shall not exceed 90p in £ (or 90% of the winnings). The Rule 4 deduction is only applied to the winnings, not to the original stake.