I’m planning to purchase a license, but as V4 is on the way and when I check the upgrade prices, shouldn’t I better wait? Isn’t there any “special” upgrade price for late ordering of an “old” version?
Actually, you’re already purchasing license for the upcoming V4. But before this new version is released you will get a valid license for V3.5.
You can find that information is the dedicated FAQ.
From June 18 until the official release of Unity 4.0 any new customers that pre-order Unity 4.0 automatically get a Unity 3.5 license for immediate use.