Reading chapter 9 of the Poser 4 manual (pages 205-230) would be a good start. Pay particular attention to page 211 and the Scrubber controls in the Timeline at the bottom of the screen.

A quick start:

You start in frame 1 of XXX (default XXX = 030, but you can set the number by clicking on it and filling in the box). Pose as you want in frame 1, then move the scrubber to frame 10 (for example, but could be frame 2 or 30), and make changes to the pose. Move the Scrubber to the next frame you want to change and repeat. 30 frames = 1 second of video.