I'm selling my bmw and I've just pulled out the V5. I'd never noticed but it simply has make BMW and engine size 4000 (it's 4398?) and no model etc? Is this just an error or is it because it is so old 1998 that the info has just not made if down the line through the owners? Or should I be worried?

I wouldn't worry too much- DVLA often miss out things or list them incorrectly. So long as your VIN matches along with your reg don't be unduly concerned. I once had a three door fiesta registered by them as five. One of those things, maybe the info didn't reach them from the dealer so they've filled in best they can.