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In contrast, defeat for Town has consigned them to relegation, with Michael Jackson's side mathematically assured to finish in the drop zone in England's third tier.
Shrewsbury were given a glimmer of hope early on, as Jon Taylor headed the hosts into a one-goal lead after five minutes.
The advantage was short-lived, however, as a brace from Washington overhauled flipped the encounter on its head after 21 minutes.
Isgrove then slotted home a third shortly before the half-hour mark, before Salop threw themselves a lifeline by reducing the arrears on the stroke of half time through Tom Bradshaw.
McQuoid rendered the miss academic, however, by adding a fourth deep into stoppage time as United keep their hopes of promotion into the Championship alive.