Probably, but IMO the relatively small difference in cost is a worthwhile "insurance premium" against the expense, the effort, and the damage to the garden of having to dig a 40m trench a 2nd time if ever the load does increase.10 or 16 would be overkill
Would you dig a 40m trench for someone in return for a payment equivalent to the difference in cost between 40m of 4/6mm² and 10/16mm²?