There were two major battles fought on 4 April 1813.
The northern one was at Salzwedel, where the Polish XIII corps attempted to take the town in order to reestablish their lines of communication with the main French Army to the south.
3rd Prussian corps held the town, and at nightfall XIII corps were forced to retreat back to Domnitz. Both corps were disordered and would be non operational throughout 5 April.
The southern battle was at Colbitz. This was a more complicated battle, which involved two French and three Prussian corps. Only one French and one Prussian corps were directly involved.
IV French corps was ordered to hold Colbitz. They came under attack from 1st Prussian corps and elements of 2nd and 4th Prussian corps.
At nightfall IV corps retreated towards Magdeburg. 1st corps entered Colbitz. Both corps were disordered and would be non operational throughout 5 April.
Apart from the four corps involved in the fighting, all other corps would be fully operational on 5 April.