This is not one of my specialist subjects and I was anticipating that one of the resident experts (Eddie, Corbyloon) would know about it.
Concentrix seem to have been engaged by HMRC since last November to check on certain working tax credit claims, presumably because there is some evidence of abuse by some individuals.
In the first instance, look at this link:
https://www.gov.uk/changes-affect-tax-credits
It says you must tell the Tax Credit Office if your working arrangements change. If you look at the list of situations under which you must inform the Office, having unpaid holiday isn't stated as one of them. However your hours will have changed in that work.
Frankly I'd have thought Concentrix have bigger fish to fry than chase after the circumstances behind 4 days unpaid. Does the letter from Concentrix made specific reference to this? Or are you just linking the two things.