there is nothing Turbotax can do to help you with the difference. Turbotax is only informed about whether an e-filed return is accepted or rejected. it gets no info on a mailed return. did you check to see that the $4200 payment has cleared your bank? the IRS should send an explanation of any changes but that will be weeks after the refund was received. you can also check the transcript of your return.
https://www.irs.gov/individuals/get-transcript
