(a) Abu Ma'shar narrates from Ibrahim (may Allah have mercy on him): When the imam does not deliver the sermon on Friday, he should lead four rak'ahs.
(b) Sa'id bin Jubayr (may Allah have mercy on him) says: The Friday prayer was four rak'ahs, but the sermon was put in place of two rak'ahs.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الجمعة / 5703