Because the two-rak‘ah obligation ends at rak‘ah two, its required final sitting occurs there. Going on without sitting for tashahhud omits a required integral; completing four cannot correct that omission.
With the sitting completed, the first two satisfy the fard and the last two count as voluntary, although intentionally delaying the salutation is not the prescribed practice.
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The Traveller’s Prayer: Journey Distance, Qasr, Residence and Make-up
Because the two-rak‘ah obligation ends at rak‘ah two, its required final sitting occurs there. Going on without sitting for tashahhud omits a required integral; completing four cannot correct that omission.
With the sitting completed, the first two satisfy the fard and the last two count as voluntary, although intentionally delaying the salutation is not the prescribed practice.