Limit data entry to one of four cells in a row
 

I have a budget spreadsheet that has columns for monthly, quarterly,
semi-annual and annual payments. I use the data from those columns to
populate a fifth column, labeled "Monthly Budget" which does the math
(quarterly*4/12, for example) based on which of the four columns is
populated. My issue is, how can I make sure that only one column is
populated on a given row? I don't want someone to enter a value in both the
monthly and quarterly columns on the same row for example. Once one of the
four columns is populated, I want to alert the user that they can't populate
another column before removing the value from the other column if they try
to do additional data entry in the row, by mistake.

I would expect a user to enter a value in the monthly column for a mortgage
payment, and in the quarterly column for a garbage bill and in the
semi-annual or annual column for an insurance bill. Right now, I'm looking
for a non-blank value in any of the four cells in the row to populate the
fifth cell using nested this IF statement:
=IF(ISBLANK(B5),IF(ISBLANK(C5),IF(ISBLANK(D5),IF(I SBLANK(E5),0,E5/12),D5/6),C5/3),B5)

What can be used to prevent more than one of the four columns in the same
row form being populated?

Limit data entry to one of four cells in a row
 

Thanks for you help. It's working.