Regulation 131.8 is the one that dictates that the bonding shall be adequate to the regs and then you go down to sections 544. and in the case of the 4mm^ then as reg 544.1.1 says the bonding must be half the size of the main earthnig conductor in a TN-S or a TT to a minimum of 6mm^ and in a PME be sized according to table 54.8 which for most domestic cases be 10mm^. So no matter which way you play this you should change the bonding.
In some case where you have a 6mm^ as in 16th edition regs it is somestimes thought that if you take a risk assement on the conductor and it as no sign of damame or burning, there could be a case to leave it and then mark it down as a deviation on the schedule of results. I'm not a great believer in this, unless it was virtually impossible to change it dues to conductor running under the floor which is covered by hugely expensive covering.
The 4mm^ though as to be changed. Perhaps you could work it into the price another way, but unless you can't you may have to walk away.
