These four months and 10 days are counted according to Islamic calendar therefore if iddat has started in in Moharram, then if Moharram has 29 days she will count it complete month and if Safar has 30 days then she has to consider it as a one month like that she has to complete 4 months and then 10 days in fifth month.
So I can’t says 4 months equal to 120 days as it depends on moon sighting in every month. Some months finished in 29 days but some have 30 days.