How many rakahs the imam prays when he gives no Friday sermon

Ayyub said: The governor of Bahrain once fell ill. He instructed a man, and he led the people in the Friday prayer; he did not deliver a sermon and led them in four rak'at. When Muhammad learned of this, he said: He acted according to the Sunnah.

Something similar is reported from Ibn Sirin.

Ibrahim said: If the imam does not deliver the sermon on Friday, he should lead four rak'at of prayer.

Al-Hasan said: If the imam does not deliver the sermon on Friday, he should lead four rak'at of prayer.

Tawus said: If the imam delivers the sermon on Friday, he should lead two rak'at; and if he does not deliver the sermon, he should lead four rak'at of prayer.

Al-Zuhri used to pray four rak'at when no sermon was delivered.

Al-Dahhak said: When no sermon is delivered, four rak'at are to be prayed.

Burd said: Makhul set out with the intention of Hajj. When he reached Tabuk, Friday came. Their imam prayed two rak'at and did not deliver a sermon. Makhul said: May Allah destroy him! He has both shortened the people's prayer and not delivered the sermon. The Friday prayer was shortened only because of the sermon.