BT lines normally have a REN ('Ringer Equivalence Number') of 4. Each device attached to the line (fortunately, except your router and/or Sky box) has its own REN, and the total can't exceed the line's REN.
So, if each of your phones only has a REN of 1, you'll be able to use all 4 of them. Most 'standard' phones have a REN of 1. Some cordless phones actually have a REN of less than 1, whereas some types of phone have RENs of, say, 1.5. You'll need to look at the labels, which should be on the base of each phone, to check the relevant RENs and to add them up.
Chris