A:
check what file system the flash drive formatted as. it is likely formatted as FAT16 which limit each file to 4GB. if so, you'll need to reformat it to file system that can handle large files. in Windows OS you can format the flash drive to NTFS or exFAT which can hold much larger file sizes. i think the Mac will have exFAT but not the NTFS format.
do note that reformatting will erase any existing files on the drive, and make sure you are formatting the flash drive and not the computer's drive!