There are two ways to look at this question: the market value and the cost of production.
In terms of market value, the answer is yes: this was the going rate for such audit reports and continued to be so until recently. Indeed, even though it is the case that there are more expert report writers in the market today, the cost of a report is, I understand, in the region of £3,000. And this does not include the cost of ATE insurance, which is now typically about £3,000 up-front, nor the cost of lead acquisition, which is now typically over £700, nor the cost of case management, which is normally billed on an hourly basis at law-firm rates.
In addition, the ATE insurer did extensive due diligence on the reports and their cost and signed-off on the figure; that is, specifically agreed that £4,000 was a fair figure and that the insurance policy would therefore cover it as an allowable disbursement.
In terms of cost of production, the answer is two-fold.
Firstly, it is irrelevant. A lawyer may spend an hour thinking and charge £1,000 for this or £100 for this, depending on their stature; arguably the cost of production has been identical in both cases, but the value may be vastly different. It is the market which determines the value.
Secondly, as it happens, in this case the cost of production was actually commensurate to the monetary cost incurred.