I installed a 4" ABS DWV riser when I poured the concrete for my basement floor. After it all cured, I cut the pipe flush with the floor, and glued in a toilet flange inside the 4" pipe (with a knock out to enable leak checking). When I filled the vent stack, water gushed from below the flange (bad glue joint?). I knocked out the removeable plastic, and stuck an inflation ball 8" down into the 4" pipe to complete my stack test, and all held fine. Now, how do I fix the bad glue joint? Ideally, I'd like to cut off the top 2-3inches of the riser including the bad flange (from the inside of the 4", not busting any concrete), find a long tail 3" insert, and some kind of flexible donut seal outside the new 3" and inside the existing (embedded in concrete) 4" riser. I have 18" of vertical 4" below the floor. Any help or advice is appreciated. Thanks