From what I can understand, after speaking to various police officers, the problems arise when there are a number of suspects and or witnesses, whose statements need to be corrobarated against each other.

By releasing people after 4 days, they can arrange to have aliases 'confirmed' or potential witnesses put under pressure.  Furthermore, when suspects caome in under the influence, the police are obliged to allow them up to 18 hours (IIRC) to recover.  This means that the time for questioning is often considerably less than the 96 hours refeerred to by the judge.