Birmingham heaped more misery on West Brom by cruising to a victory which leaves their local rivals bottom of the Premiership at Christmas.
Robbie Savage stroked in a penalty to put the home side ahead after Darren Purse's challenge on Clinton Morrison.
West Brom's only threat had come with Zoltan Gera hitting the crossbar.
Morrison picked his spot to add Blues' second before Emile Heskey's angled shot and Darren Anderton's defelcted free-kick completed an easy win.
West Brom boss Bryan Robson had dropped Robert Earnshaw to the bench for the game and started with Kanu as the lone striker with Thomas Gaardsoe as an extra body in midfield.
But the move badly backfired after Birmingham
went in front through a Savage penalty after Purse was harshly adjudged to have held Morrison as the Blues striker turned to try and latch onto a Heskey header.
Savage slotted home just a minute after referee Mike Riley had left the Blues camp fuming at being denied a more blatant spot-kick after David Dunn was brought down by Bernt Haas.
Blues strikers Morrison (left) and Heskey each scored
West Brom worked a rare opening as they looked to equalise but their fortunes were summed up by Gera's shot as he feel back being touched onto the crossbar by keeper Maik Taylor.
The goal had served to add to Birmingham's confidence and they had another shout for a penalty waved away before adding a second.
Dunn caught out a snoozing West Brom defence with a quickly-taken free-kick which Darren Carter slid a ball across to Morrison who was allowed plenty of time and space to slot in.
Birmingham were oozing confidence against a Baggies side in disarray and cruised into a three-goal lead when Heskey angled in a shot after Dunn's pass.
The visitors improved slightly after the break but not enough to cause the St Andrews side cause for concern.
Birmingham's main worry was not to lose concentration and they were given a warning against complacency when gifting Earnshaw a chance.
Blues defender Kenny Cunningham's back-pass found Earnshaw with Taylor off his line but the keeper recovered the situation to save a tame shot from the Baggies striker.
No club bottom at Christmas has managed to escape relegation since the Premiership began and West Brom will struggle to change that statistic on the form they showed against Birmingham.
They were further demoralised when Anderton's late free-kick was deflected in for Birmingham's fourth.
Birmingham: Maik Taylor, Tebily, Cunningham, Upson, Lazaridis,
Johnson (Anderton 78), Savage (Clapham 87), Carter, Dunn,
Morrison, Heskey (Gray 83).
Subs Not Used: Vaesen, Clemence.
Goals: Savage 4 pen, Morrison 23, Heskey 30, Anderton 80.
West Brom: Hoult, Haas (Wallwork 75), Scimeca, Purse, Clement,
Gera (Earnshaw 45), Koumas, Gaardsoe, Johnson, Greening, Kanu.
Subs Not Used: Horsfield, Dyer, Kuszczak.
Booked: Koumas, Haas, Greening, Gaardsoe.
Ref: M Riley (W Yorkshire).