I would hazard a guess that this is due to there being a low fare bucket for WTP for fewer than 4 people. It means everyone gets booked into a higher fare bucket.

Booking all 4 people separately will not save £2000 (500 x 4), because some are likely to have to be booked into a higher fare bucket. In terms of how many will book into a lower fare bucket, you will have to experiment with it (unless you've already done that and you don't get the lower fare until you got down to just 1 person).

In addition my guess is that CW fare is inflexible, whereas WTP probably is.

I suggest playing around with the number of seats to book to see how many you can get WTP at a lower price and then make a decision.