The USB stick which gets formatted during creation of the boot media is formatted to FAT32. There is a limitation of a maximum 4Gb file size limit under FAT32.
There are two ways to address this issue when storing large image files (where the total image size is beyond 4Gb).
After the boot disk creation of the USB flash media, use the Microsoft Windows command CONVERT to convert the file format from FAT32 to NTFS.
Leave the USB media as FAT32 and specify that Active@ Disk Image split the image into 2Gb or 4Gb image chucks during creation of an image.