My guess is that your last 4 digits (after the 400) show your purchase # for BWV. If yours is 4001007 - you would be BWV purchaser #1007. If 4002475 you would be purchaser # 2475 at BWV. If 4000007 you would be BWV's seventh purchaser ... but I could be wrong too.
Based on my experience buying OKW in 1993 - any direct purchase made with that same Use Year would have the same Member number and the same base contract number - the digits are the decimal indicates the number of "direct" individual contracts you have purchased for that Use Year (regardless of the resort). Our original contracts were listed as 8XXX.0, 8XXX.1, 8XXX.2, etc. But at this time there may be purchasers with enough add-on contracts that they are using xxxx.000, xxxx.001, xxxx.012, etc.
If you also purchase(d) a resale with the same Use Year and titled exactly the same, I expect that the contract # would be based on the resale contract even though it is being added to a different Member # than the original purchase for that contract.
Again - these are my assumptions based on the direct and resale purchases I made between 1993 and 2000.