The exact limit is 4,294,967,296 Bytes a.k.a. 4.29 GB a.k.a. 4.0 GiB.
You can use Check4GB to check if the partitions are inside the limit.
You can use FixHDDSize to limit the harddrive to 4 GiB so that you don't need to care when creating or moving partitions.
You can also manually limit the harddrive to 4 GiB if you overtype the geometry by these values:
cylinders = 8322
heads = 16
blocks per track = 63
blocks per cylinder = 1008