This is a safety margin. Split4G creates slightly smaller parts to ensure that each piece is safely under the strict 4GB limit of the FAT32 file system, preventing any "file too large" errors during the transfer to the USB drive.
Choose where you want the split files to be saved (ideally, directly to your USB drive or a dedicated "PS3ISO" folder).
: Click the splitting button. The tool will scan for files over 4GB and divide them. Rejoin on PS3