In the 3D Layer Configuration, set all slices except the 3rd to 'fixed', and change the Distance from Lower Slice to 1 m (Standard user interface). In the Classic user interface, you cannot set a distance from the slice below, just from above. Here you would have to add slice 3 from scratch, specifiying a distance from the slice below while adding the new slice. Another option would be to export all elevations from slice 4, add 1 m to all elevations externally and import them again for slice 3.