Albion suffered a second-half horror show at Norwich as the Canaries ran riot at Carrow Road.
Ben Foster’s clanger and Gareth McAuley’s own goal gifted the hosts a comfortable win after Robert Snodgrass opened the scoring in the first half.
McAuley has now scored own goals for Norwich while playing for Ipswich, Leicester and Albion.
Jonny Howson added a brilliant fourth for the Canaries in the final minute as they guaranteed their Premier League status next season.
It left Albion on the back of three straight defeats and they sit eighth ahead of next Sunday’s final game against champions Manchester United.
Foster, Steven Reid, Goran Popov, Shane Long and McAuley all returned following the 1-0 defeat to Manchester City.
Chris Brunt was on the bench after a knee injury while Norwich recalled goalkeeper John Ruddy after six months out with a torn thigh.
Norwich, who knew a win would secure survival after Villa’s defeat to Chelsea yesterday, started with pace and purpose but didn’t seriously threaten Albion.
The hosts dominated possession but lacked the quality final ball to open Albion up and despite their bluster.
The Canaries finally tested Foster on 24 minutes when Anthony Pilkington spun in the area and the keeper palmed his effort away with Olsson ushering out for a corner.
The corner was only half cleared though and from the next attack City took the lead.
Holt’s deep cross was headed down by Ryan Bennett for Wes Hoolahan and despite Foster saving at the Irishman’s feet Snodgrass steered the rebound into the roof of the net.
It raised the Carrow Road roof as Albion started to struggle with Norwich’s intensity, despite looking assured in possession.
The Baggies looked to kill the Norwich heat when they had the ball and Lukaku was menacing when he roamed wide.
But Norwich almost grabbed a second when Foster tipped over Hoolahan’s brilliant drive five minutes before the break.
Brunt and Billy Jones replaced the ineffective Markus Rosenberg and Reid at the interval but Norwich continued to press and Foster cleared from Snodgrass’ feet after Pilkington’s knock down.
Olsson was then booked for chopping down Holt before Snodgrass broke only to see his shot charged down on 56 minutes.
Norwich pressed and Albion escaped when Foster spilled Bradley Johnson’s cross into the packed area.
But there was no let-off when Foster and Olsson dropped a clanger to gift Norwich a second on 61 minutes.
Holt swapped passes with Hoolahan to arrow in on goal but with Olsson covering there seemed little danger.
But Foster raced out and only sliced his clearance backwards to allow Holt to tap into an empty net.
And two minutes later Norwich made it 3-0 when McAuley headed Snodgrass’ free kick into his own net under pressure from Bennett.
Albion at least stuttered into life and Lukaku and Jones tested Ruddy with cursory efforts before Long’s low effort was gathered with 15 minutes left.
Norwich were still hunting goals though and Foster denied Pilkington with a fine low save with six minutes remaining.
And the hosts wrapped it up in stunning style when the impressive Howson added a fourth, curling in from 20-yards in the final minute.