Hmm... so they'll sterilise you but not your husband, because you have one more biological kid than he does?
So they're basing this decision on the premise that your husband will want to impregnate other women in his future, rather than staying faithful to you. And that you, as the woman, are more likely to have had enough of procreating and will instead want to devote what's left of your energies to child-rearing. Whereas your husband, not being tied down by the physical constraints of pregnancy and breast-feeding, will not want to be tied down with you, but will want to be out there spreading his seed around the female population.
I guess they're basing this on statistics, but surely it'd be more constructive to view you together as a unit, since this is about family planning, and do what's best for you both, rather than acting on the assumption you're unlikely to stay together!