What the post office clerk is probably referring to is that you just miss the annual tax rise (Band K was £215 up to April 2010, then £245 from May 2010 onwards). It's not related to the year of your car (04).
The only thing that makes a difference to which Band you're in is whether the car was registered before 23 March 2006:
- Band K (201-225g/km) - for 2009 & 2010 includes cars that have a CO2 figure over 225g/km but were registered before 23 March 2006: £245
- Band M (Over 255g/km): £435
There is no official statement on what will happen in 2011, but clearly one possibility is that all 260s will be taxed as Band M, regardless of registration date.