ALBION suffered consecutive defeats for the first time this season as a five-game winning streak at The Hawthorns came to an end against Derby County.
Jack Marriott, Tom Lawrence, Harry Wilson and Scott Malone were all on target as the visitors ran out convincing winners on Wednesday evening despite Jay Rodriguez's late consolation.
It was a disappointing night for the Baggies which saw Kieran Gibbs and Dwight Gayle forced off at half-time owing to injury.
Darren Moore made two changes from defeat at Wigan Athletic - both enforced. Tosin Adarabioyo replaced Ahmed Hegazi who reported a "tight hamstring" prior to kick-off. Jake Livermore's suspension meant Sam Field earned his first league start of the season.
And it was the hosts who had the first sight of goal, Rodriguez firing a low shot left of Scott Carson's near post.
But it was to be the Albion man's error that gifted the visitors a 10th-minute lead, Rodriguez's loose backpass converted by Marriott.
It went from bad to worse for the Baggies as Wales international Lawrence drilled in off the woodwork to double the Rams' lead just five minutes later.
Moore's men would have found themselves three goals down before half-time but for Sam Johnstone. The Baggies number one saved well from Malone, Tom Huddlestone and Jayden Bogle.
Keen to atone for his earlier mistake, Rodriguez struck a fierce half-volley straight at Carson, before the Derby 'keeper was quick off his line to intercept a pass intended for Gayle - Albion's best opening of the first 45 minutes.
Albion were forced into a double substitution at the interval after Gibbs and Gayle picked up slight knocks - Tyrone Mears and Hal Robson-Kanu their replacements.
But Frank Lampard's side put the game beyond doubt just six minutes after the restart, Wilson punching the ball past an unsighted Johnstone.
Harvey Barnes andField tried their luck from distance in an effort to claw their side back into the tie, with Scott Malone denying Robson-Kanu a certain goal with 20 minutes to play.
It was to be one of those nights for the Baggies as Malone found himself one-on-one with Johnstone at the other end just moments later. He made it four with a calm finish.
Rodriguez slotted a late response but the Baggies must regroup ahead of another Hawthorns clash with Blackburn Rovers on Saturday.
ALBION: Johnstone; Dawson, Adarabioyo, Bartley; Phillips (Morrison 62), Field, Brunt, Gibbs (Mears HT); Barnes; Rodriguez, Gayle (Robson-Kanu HT).
Subs: Myhill, Sako, Edwards, Hoolahan.