I’m betting that your disk is formatted using the FAT32 scheme. The maximum file size under that scheme is 4GB.
from “Segment settings”:
“If you are exporting a movie that is larger than the maximum file size for your hard disk formatting scheme, then you can set the Segment Video-only Movie Files At value to a value under this maximum. Hard disks formatted for Windows can be formatted using the FAT, FAT32, or NTFS scheme. The maximum file size in the FAT scheme is 2 GB, and the maximum file size in the FAT32 scheme is 4 GB. The maximum size for a file for NTFS is very large (approximately 16 terabytes), so you are unlikely to reach this limit with a single movie.”
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Todd Kopriva, Adobe Systems Incorporated
putting the ‘T’ back in ‘RTFM’ : After Effects Help on the Web
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