The Rule 4 deduction is an Industry standard where a quoted deduction is applied for every 1.00 unit of currency won.
When bookmakers offer odds on an event, they do so expecting all participants to take part. When a participant is withdrawn from an event a Rule 4 deduction is applied to all bets placed before the withdrawal, to reflect the reduced amount of competitors.
The size of the deduction is dependent on the odds of the withdrawn participant(s) at its time of withdrawal, the shorter the odds the larger the Rule 4 deduction.