The Blades remain clear at the top of the Professional Development League after a nine-goal thriller at Shirecliffe, in which Jordan Hallam, Oliver Greaves and Tyler Smith all claimed two each.
United were two up inside seven minutes, with Greaves claiming both, netting crosses from Hallam and Tom Charlesworth. Cardiff pulled a goal back but Greaves turned provider for Hallam to make it 3-1 on 36 minutes.
Cardiff again reduced the arrears but United made a bright start to the second half. Another Greaves cross saw Hallam flick on for Tyler Smith to score. And moments later Smith added his second with a deflection.
A third Cardiff goal on 67 minutes gave them slight hope but the Blades rounded the scoring off on 72 minutes when good pressing by Burton saw Hallam benefit.
Coach Derek Geary said afterwards: "I was delighted with the attacking aspect of our game, the movement, combination play and clinical finishing of the front four was excellent but I was disappointed with the defending for their goals, especially as it is the first goals we have conceded at home this season."
Blades: Perryman, Charlesworth, Ferguson, Graham (Ford 48), Cantrill, Slater (Hawkes 76), Gilmour (Mallon 72), Smith, Greaves, Hallam, Burton.
Unused: Parkhouse, Warhurst.