How many frames do you want to grab? I'd play it on my computer, pause it at the appropriate point and do a screen grab. Any photo editing software can be used to visually tag the frames how ever you want. Or am I missing the brief by a country mile...
Need to capture EVERY frame, in sequence, over a four second period. That's, what, 100 frames or so? And they need to be indexed so there's no disputing the sequence.
This is for a Court issue (industrial accident) so the timestamps need to be irrefutable.