Each additional qualifying child adds US$15,000 in processing fee plus a US$1 million gift, so your household moves from US$3,045,000 for three people to US$4,060,000 for four. Those are program amounts only. Government visa and medical charges and professional fees sit on top and are not part of those totals.
Reading the totals without over-reading the refund position
The US$15,000 per person is the amount the instructions describe as non-refundable; that statement is not a general rule about every other charge, and it is not a promise that anything else is returnable either. Nor is the payment schedule a single lump sum: the application fee comes at one stage and the gift only when you are instructed to make it. A written quote from your professionals is worth having before you commit.
The official page describes an individual route in which the applicant makes an unrestricted gift to the U.S. government after vetting and pays a nonrefundable processing fee, and a corporate route in which a company makes a larger gift for a sponsored employee. Corporate sponsorship at US$2 million per employee with annual maintenance and transfer charges does not apply to a retiree with no employer.