I am trying to do the following: 
On a contact record, if the Mobile Phone field is not blank, use it to populate the Phone in Use field on the same contact record. If the Mobile Phone field is blank, then use the Work Phone field to populate the Phone in Use field. If the Work Phone field is blank, then use the Home Phone field to populate the Phone in Use field. If the Home Phone field is blank, then use Other Phone field to populate the Phone in Use field. If the Other Phone field is blank, the Phone in use field remains blank. 

I created the formula below, but I am getting an error I don't know how to fix: "Error: Incorrect parameter type for function 'IF()'. Expected Boolean, received Text"

IF(NOT(ISBLANK(MobilePhone)), Phone_In_Use__c = MobilePhone,
IF (NOT(ISBLANK(npe01__WorkPhone__c)), Phone_In_Use__c =  npe01__WorkPhone__c,
IF (NOT(ISBLANK(HomePhone)), Phone_In_Use__c =  HomePhone,
IF (NOT(ISBLANK(OtherPhone)), Phone_In_Use__c =  OtherPhone,
" "
))))