Focussing
The Sprint Scan 4000 has a focus capability in their hardware. This focussing can be applied using SilverFast by simply enabling the focus button. This generally brings the focus to a ballpark range which can be extended when using it with the filters below.

Focus not enough? Try unsharp masking
Sometimes the internal focus of the hardware is not enough. If the image still appears blurry or you are trying to enhance the original for the same reason, you can use the extra sharpening capabilities of SilverFast.
There are a few ways to do this. The normal way is to use the Auto Sharpen filters. There are different levels, lore Auto Sharpen and Less Auto Sharpen. These will enhance the image even more during final scan time.
Yet another way is to use SilverFast's powerful unsharp masking filter. With this tool, an unsharp mask algorithm is applied to the final scan. This has customizable features and almost guaranteees your image will be much improved in detail.