The appearance of IV corps has caught Blucher by surprise, and he is slow to deploy his army to meet the new threat.
1st corps are dispatched to take the high ground south of the farm. 3rd corps move from the farm to face the Celle road. 4th corps are ordered to garrison the farm as their left flank and deploy between the farm and the high ground allocated to 1st corps.
The Prussians started the battle with only one cavalry brigade, and lost that in the early moves. So their deployment is much disrupted by the two French cavalry brigades.
Davout has two corps available on the Celle road, but both of them are below half strength. One has already started to retire to Celle, and is now marching back. He can not take any offensive action until both are ready to advance.
So throughout this period IV corps set the pace. Reynier is determined to attack the Prussians, not just join forces with Davout. To this end he intends to take the farm, and split the enemy in half. He has deployed his corps to take the farm with his right wing, and hit 4th corps before they can deploy. He is unaware that 1st corps have orders to take the hill which overlooks his left flank.
As IV corps approach the farm, 1st corps deploy on the hill within canister range of their left flank. Reynier orders his corps to swing to the right, which will bring him nearer to Davout. His left wing will now attack the farm, and his right wing hit 3rd corps as they deploy to meet Davout.
Davout joins Reynier and orders him to attack the farm. 1st corps artillery continue to fire on the nearest brigade and 4th corps artillery deploy to support the farm. By the time the two French brigades reach the farm they have already suffered casualties, and are soon driven out of the farm in rout.
All now depends on the right wing of IV corps. The cuirassier brigade is held in check by a Prussian square, but the remaining two infantry brigades now advance to attack. The artillery is unable to support, as they have had to redeploy from the left to the right flank.
As IV corps attack starts Davout receives a report that Prussian cavalry have been sighted approaching Celle. The loss of that town would cut his only line of retreat. He surmises that the cavalry belong to 2nd Prussian corps, and if they take Celle he would be caught between two Prussian forces.
He must decide whether to order an immediate withdrawal and secure his lines of communication, or to advance to support Reynier and hopefully smash Blucher. He orders an advance on the farm. It is now all or nothing.