I have 4 fields - Category1, Category2, Category3, Category4

I have created a formula field which concatenates all these into 1 single field and shows in the new field called TotalCategories

Total Categories = "Category1:" & Category_1__c & ";Category2:" & Category_2__c & ";Category3:" & Category_3__c & ";Category4" & Category_4__c

This is absolutely fine. But when only Category1 is filled.. Total Categories should show only Category1:"Category_1__c". Can anyone help .. how to do ? I tried using 'Case' but no use.