platinum dongle/ whats it is worth?

I doubt it is worth anything much...I upgraded to v6 just before the buyout, and was involved in a bad car accident before I could install, and didn't get to it for almost a year. In the meantime, the whole emagic sale happened, v7 came out, etc...I was at a loss for how and with whom to register, etc. When I finally got ahold of someone in Germany who referred me to someone in Cupertino, they told me to not even bother sending in my emagic dongle. So I still have mine, too.

I figure I could install 6 on a powerbook for gigs, but other than that it's a paperweight.

The Logic 4 dongle is worth pretty much nothing. When the Upgrade to Logic 6/7 was available, it sold for up to $50, because it was a cheap way to buy the latest version of Logic. But now - because this Upgrade is no longer available - people don't even get $10 for a Logic Platinum 4.5.

If you already have Logic 5 or later with an XSKey, it is a totally different thing. Here you only have to be aware that some people probably still try to sell old demo XSKeys for good money at eBay. These can't - of course - not be updated.