This depends on how long you have had your prefilters in. If they have been in a while then you can't go wrong by changing everything, especially if they are 10" prefilters. If you use alot of water then you are better to change 10" prefilters more regularily, even monthly in some instances.
The carbon block/GAC filter is the most important IMHO as that prevent chlorine from damaging the r/o membrane, which on a 4040 is well over £200.
Change resin as well.