Q: If a husband and wife lived apart for more than four months, is it necessary to remarry before being together again?
A: Talaaq does not take effect by a wife being separated from a husband (and vice versa) even though this period may be for a number of years.
Yes, if the husband had taken an oath not to approach his wife for a period of four months or more (or perpetually), then upon the expiry of four months a Talaaq will take effect. This, in the Shariah is called "Eelaa', which is mentioned in the Quraan Shareef and is specific to the husband taking an oath in this regard. If no oath or vow was taken, then a Talaaq will not take effect by mere separation.
And Allah Ta’ala Knows Best
M.D. Mangera (Mufti)