Substitute David Dunn continued his slow rehabilitation from an 18-month spell plagued with back and hamstring injuries by grabbing the 67th-minute equaliser that earned Blues the second chance that they deserved for their second-half performance alone.
Blues boss Steve Bruce was forced into making two changes to the side that had thrashed Portsmouth last week. Matthew Upson was serving a one game suspension whilst skipper Kenny Cunningham was ruled out with a groin injury.
With Martin Taylor also ruled out with an ankle injury, an untried central defensive partnership of Olivier Tebily and the managers son Alex was therefore chosen to face the Coca-Cola Championship table-toppers.
Nineteen-year-old striker Shane Long put Reading in front with their first and only chance of a somewhat scrappy first half.
John Oster and James Harper combined to help the ball into the youngster's path, and he kept his head to crack home a clinical shot that easily beat Blues keeper Maik Taylor at his near post.
Reading were slick in the build-up and confident at the back, but created few chances to add to their opener.
Blues though, created even less and manager Steve Bruce, who lost his voice during the match, was forced into desperation measures as he looked to turn the tie.
Cometh the hour, cometh the triple substitution by Bruce that did help them get a foothold in the game.
Dunn, Mikael Forssell and Neil Kilkenny came on to replace the ineffective Emile Heskey, Jiri Jarosik and Muzzy Izzet and within minutes they had Reading on the back foot.
Jermaine Pennant's cross allowed Dunn to chest down and fire over the bar and, seconds later, Dunn was denied by keeper Graham Stack who kept out his diving header from another Pennant cross.
But Dunn made it third time lucky on 67 minutes when he found himself in space to fire home at the far post after Chris Sutton flicked on Mario Melchiot's long throw.
Blues, relieved to at least be in the hat for Monday's fifth round draw, came within a whisker of completing the comeback in the last minute as Ibrahima Sonko felled Forssell on the edge of the penalty area.
Pennant's free-kick was heading for the net, but the former Arsenal man was denied by his one-time Highbury team-mate Stack, who made a tremendous save at full stretch to ensure a return tie at St Andrew's.