Really depends on the specific situation - e.g. how long was the relationship/marriage/civil-partnership, how well do the separating couple get on in spite of the relationship breakdown, are there kids, how complicated is the situation with regard to assets/means etc. etc.is it complicated
Possibly nothing if you (and your legal representatives) both agree on the effective date of separation (which may not mean actually living apart - just the end of normal marital/partnership arrangements). But if you don't agree on this date then that's a different matter and worst case might be the date you both started living in separate locations if applicable.what do you need to produce as evidence that you have been living apart for four years when going a divorce?