Re; the softness - It may not be your lenses at all. I have also used a 4490 to scan, and had major issues with the holders not keeping the corners flat. The holder is also a fixed height and this used to frustrate my attempts to get a sharp image (even though prints made from negatives previously were razor sharp). Some emulsions scanned well and others did not (variations in thickness?). The point I am making is that the scanner did not always get the correct focus plane.
I did toy with getting a height adjustable holder from
this guy, however he indicated back then that he was only doing 645 holders.
In the end a Minolta dimage elite II came up locally and I was able to buy it - It has been well and truly worth the money. I still use the 4490 for scanning 645 negatives though and it is pretty good.