Why would you spend your time and energy (maybe money even) on something that can be bypassed so easily?

From a surfers point of view I would only be annoyed if I have to clear my cookies after 4 previews to see the next ones. Cookies are no mystery anymore and most surfers know deal with them if necessary.

Also it would make me think what kind of "limitations" are hidden inside. It's like window shopping where the shop owner pulls curtains over his remaining windows once I've looked at the first three windows. Would you go inside and buy there?

If you want to limit the number of preview videos on your tour (what's the reason for a tour?), just offer four videos and only show screenshots of the rest. And make those screens clickable so they link to the signup page instead of telling a potential customer that there are more videos, but he's been a naughty boy because he wanted to see more than four videos.