We have been giving Pediarix (DTaP-HepB-IPV) to children who are overdue for DTaP #4, IPV #3, and HepB #3. Is this an acceptable practice?

No. Pediarix is intended to be used only for doses 1, 2, or 3 of the DTaP primary series; consequently, using Pediarix for DTaP #4 is off-label and not recommended. You should take measures to prevent this error in the future. The DTaP, IPV, and HepB doses given in this scenario do not need to be repeated as long as you met the recommended minimum intervals for each vaccine component (DTaP, IPV, HepB). If you did meet the minimum intervals, the doses should be counted as valid.