How long is this spindle? 24 inches or 6 inches? It makes a difference as to where the midpoint is
If for some reason there is no way to improve this design, then put one pair of bearings in the middle and the other pair at the drive end. The further back from the business end that you put the bearings, the more load is transferred to the rear bearings. So to get the maximum stiffness of a poor design, that is what I would recommend.