You can do that one of two ways:
1) You can ring calls to an extension list (will ring all the phones but the call will not forward any further)
2) You can create a hunt group with those 4 phones as members. If you add them as individual members, they will only ring one phone at a time. If you create a hunt group members extension list with those 4 phones and then make that extension list a member of the hunt group. It will ring all the phones at once. Hunt groups give you a bit more flexibility as to what happens to the call if not answered.