Rule 4 allows bookies (and betfair) to make a deduction on odds offered in the event of a non runner. If you back a horse at 8 and then the favourite were to be withdrawn after your bet this of course alters your horses chance of winning. This is where rule 4 comes in and the bookie will deduct a % from the odds, this can vary from book to book but is usually between 15 - 20%. Betfair will also make a deduction from the odds you layed at
If you back at 8 and lay at 6.5, check that there are the same number of runners on each card as you could be laying on a market that's reformed after a non runner but backing before the non runner.
If both racecards show the same number of horses running then this example is a no brainer as even after rule 4 deductions you still have a decent arb
If the odds were much closer, say 8 and 7.6, then depending on each bookies method of deduction an arb could turn into a loss, or vice versa.
If your horse wins at the bookie then you will be paid out at the reduced odds and you will pay at reduced odds at the exchange (Usually slightly not in your favour). If your horse doesn't win then you will collect your backers full stake as usual (always in your favour)
