If a bookmaker gives a horse 4/5 odds to win means the horse is an odds-on favorite (if you bet on this horse, your initial stake is higher than the potential profit). A winning $10 bet with these odds will make you a profit of $8. Add this profit to your initial stake to find out how much your total return will be.
Total return formula of 4/5 horse racing odds with a $10 stake: $10 initial stake amount x (4/5) = $8 profit + $10 original stake = $18 total return