It is funny and useful IMHO. New event IDs from Security Event Log in Vista appear to have theirs analogs in more old systems (no wonder, actually), but they are away from those exactly by 4096.
It is described why it was done here. In short, great changes were introduced into the content of the events, so there was a need to distinguish between pre-Vista and Vista events. Now as in the source code numbers are represented as hex values they added not 5000 or 6000, but 0x1000=4096