The number of movies that will fit on a 4GB flash drive depends on the encoding method, resolution and movie length. Highly compressed movies with low resolutions have smaller file sizes, so you can fit numerous movies on the same flash drive. However, high-resolution movies with low compression may be too large to fit on a 4GB flash drive at all.

Actual Storage Space

Flash drive manufacturers typically express capacities under the false assumption that there are 1,000 bytes to a kilobyte, 1,000 kilobytes to a megabyte and 1,000 megabytes to a gigabyte. However, computers actually use 1,024 as the conversion between units. Therefore, your 4GB flash drive is actually limited to around 3,814MB, or 3.73GB.

Crunching the Numbers

According to Apple, a standard-definition movie averages about 1.5GB in size, so only two such files would fit within the 3.73GB space of a 4GB flash drive, leaving a little over 700MB for other files. Apple claims a high-definition movie is often 4GB, so you may or may not be able to fit a single HD movie on the drive, depending on the actual file size. Many people encode movies specifically to fit on a single 700MB CD, so if you're copying those movie files, you'll be able to fit up to five movies on the flash drive.