box 4, AP1 form - what figure if partially inherited, partially bought

Our deceased parent's house is to be transferred to one of the beneficiaries. (We've worked with a probate solicitor, all inheritance tax has been paid, probate granted, proper valuations obtained, and confirmed no capital gains tax.)
The value of the property is greater than the beneficiary's inheritance. Therefore they are paying some cash too, which is approximately 40% of the total value of the property.

Please can you advise what figure is correct in box 4 of the AP1 form.
a) the amount of cash paid by the beneficiary,
or
b) the total value of the property,
or
c) some other answer?
Would I need to add any other detail so that the form is accurate or can be understood and processed properly?

Many thanks for your help!