tes2cub

tes2cub is a mis-named program that will let you extract a single volume from a 4D file and save it as a 3D file. Note that vbconv will allow you to do the same thing somewhat more flexibly, but tes2cub is still sometimes easier to use. Here's the help:

VoxBo tes2cub (v1.8.5/Jun 14 2011)
usage:
  tes2cub <tesname> <cubname> [imagenum]