IHTM10702 | Corrective account (C4): when is an account required (England, Wales and Northern Ireland)?
From HM Revenue & Customs · Inheritance Tax Manual
Strictly speaking a C4 should be completed in all cases where there is an amendment to the account.
You can insist on a C4 where:
we are repaying money but we have any doubts that the repayment is due,
there are lots of amendments or they are complicated,
more tax is payable after a Form Cap A 5C () has been sent to us, or
assets that were given an estimated value in a reduced account () are redirected to a chargeable beneficiary ().
If you have to ask for a form C4 and there is unpaid tax not in dispute, you might want to:
raise provisional calculations, or
suggest making a payment on account.