Yes, you can take your USB-eLicenser from your PC to Mac, and update your Cubase 4 licence to the Cubase 8 licence. It doesn't matter, if you will activate this update licence on your PC or Mac.
And your second question. Yes, BUT... In theory, you are allowed to use Cubase 4 application with your Cubase 8 licence. But you should update your eLCC application. If you will not update the eLCC, it's possible the Cubase 8 licence will be not recognised, and will be displayed as "Unknown licence" on the system with the old eLCC (so then, you can't run your Cubase on this system, because there is no known licence). Now, the tricky part is: which Windows do you use? If you are using Windows 8 or Windows 7, then it's not problem at all (you will just update eLCC to the latest version: 126.96.36.1993 at this moment). But if you are using Windows XP, the newest eLCC is not officially compatible with Windows XP. So it's possible, it will not work. Then, you could try to install the latest eLCC on your Windows XP (and hope, it works), or you can keep the latest Win XP eLCC version, and hope, this version will recognise Cubase 8 licence. If you are using Win XP (I hope not), then I would recommend to try to update your eLCC before you activate your Cubase 8 Upgrade, and check, if the eLCC has been installed, and work as expected. If yes, then I would upgrade the licence.
Sorry for the long story... ;-)