Capital Allowances

Hi. I have a question regarding capital allowances. It is as follows:-

Elizabeth has tax written down value on her main pool of plant and machinery of £16,000 on 6 April 2008. In the year to 5 April 2009 she bought a car for £8,000 (no non-business use) and she disposed of plant which originally cost £4,000 for £6,000.

Calculate the maximum capital allowances claim for the year.

The answer given is as follows:-

Main Pool Allowances
£ £
Two v b/f £16,000
Addition (not qualifying for (AIA) £8,000
Less Disposal (limited to cost) (4,000)
20,000
WDA@20% (4,000) 4,000
TWDV c/f 6,000
Maximum Capital Allowances Claim 4,000

I can understand where all the calculations come from except the TWDV c/f one. I make it £16,000. I must be missing something somewhere. Someone please help! Thanks.

Matt.