We had only 3 dependents listed in the 4a and 4c lines but what appears to have been missing was the amounts sent by the IRS directly to our account which we didn't get a tax form for. After reconciling those amounts with a TurboTax expert we were able to figure out where the deficit occurred. So my advice for anyone seeing this in future: look at your bank statements from last year to see if there were other payments sent by the IRS (especially from July to December and one time in March).
