Dunn continued his slow rehabilitation from an 18-month spell plagued with back and hamstring injuries by grabbing the 67th-minute equaliser that earned Birmingham a second chance they deserved for their second-half performance alone.
Their first-half display barely warranted mention, as Reading took control despite leaving out seven of the first-choice 11 that have helped them run away with the Championship this season.
Nineteen-year-old striker Shane Long put them in front as Reading clinically accepted their first and only chance of the 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.
Reading were slick in the build-up and confident at the back, but created few chances to add to their opener.
Birmingham 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.
Birmingham, 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.