If I understand your question correctly you place 4 slides in a transparency adapter, enable Multi Crop and then Vuescan recognizes (erroneously) that you want to scan 8 slides. Subsequently it starts scanning all 8 regions where it thinks slides are present.

To only scan the four slides follow these steps:

1. Enable Batch Scan in the Input tab.
2. Choose All in the Batch Scan options drop-down menu.
3. Do a preview scan.
4. After that choose List instead of All in the Batch Scan options.
5. Now you have to find out what frame numbers the actual slides have. You can find this by varying the Frame number in the Input tab and noting the frame numbers that actually select a slide. Let's assume Vuescan recognized your 4 slides under frame #1, #4, #5 and #8.
6. Now in the text box next to the Batch List option write down 1,4,5,8. This directs Vuescan to only scan these frames.

You can find more information on the commands in the Vuescan manual, especially on page 57 where the Batch List option is explained.

As my scanner is not supported on my OS anymore I don't have Vuescan installed to show screenshots, but these steps + the manual should be enough.