A system bet allows you to bet on several accumulator bets at once. It also allows you to earn your winnings even if one or more of the wagers in your multiple bet lose.
A 4/7 system consists of 35 bets which transpire from 7 picks -- thirty-five 4-pick multis (parlays). At least four of the seven picks must be correct to gain some winnings - the exact amount of the winnings depends on how many of the predictions prove correct.