If a bottle has an age statement i.e. 15 years old, then every drop must be that age before bottling but not necessary before blending if the whisky is put back into casks for further maturation.
Although they name 4 malts I would hazard that they use at least 15 to 20 in the vatting and that the 4 malts named are because they are the ones that Diageo want to promote. Of the 4 named all are blending malts with possibly Talisker being more for a single malt. Remember nearly all single malt distilleries sell to blenders as their single malt sales normally are not sufficient to make them profitable.