Is it permissible to count the four Qirats left for the family—which were intended to guarantee rights and avoid problems—with the intention of Zakat, in order to pay off a portion of the Zakat due now?

If the waiver of the land was not genuine, but a stratagem to secure a right that could only be obtained through this ruse, then you may retract the waiver. If it necessitates a judicial ruling, the stratagem will not benefit you unless you have witnesses (an attestation) that the waiver was not genuine. Ibn Farhun stated in "Tabsira al-Hukkam" that a settlement attested secretly to deny a right is not binding if the denial and the right are proven. Mutarrif and Ibn Muzayyin said that a secret attestation is only beneficial against one from whom justice cannot be obtained, such as a ruler or an oppressive person.

Regarding the disputed land, giving it to your brother's sons does not suffice for Zakat (definition) for two reasons: First, Zakat must be from the same type of wealth being zakated, and one does not deviate from this unless there is an overriding benefit for the poor. Second, considering this land, which is still disputed, as part of Zakat is a deception concerning Zakat and an attempt to benefit from it, which is neither permissible nor sufficient.