I think you misunderstand the question you are asking. I presume the separation standards you are referring to are for vortex wake. This doesn't bear any relation to the spacings provided by the approach controller. If the Tower was only ever given four miles on final then nothing would ever depart. Approach control co-ordinate with the Tower to arrange suitable gaps for departing traffic but even without this gap you would be unlikely to see them closer than six miles on final. Obviously at mega busy units such as Heathrow with one r-way for arrivals and one for departures then the vortex could well then become the spacing on final but this is in a slightly different league from your average aerodrome. At units with unusual taxiway arrangements or long backtracks this is obviously taken into account by the approach controller and then just requires good judgement from the tower controller to utilise the gaps. Hope this helps.