Data Validation with Custom function and message box display

Hello there,

I have four cells c1 thru c4. The SUM of these cells must equal 1. I want to display an error message "sum of cells not equal to 1.0" but only when data has been entered in all four cells.
for instance =SUM(C1:C4, "<>1") should display the error message. Is there a way to write the formula with an IF statement and still use data validation with error message box?

Assumptions:
Will ignore blanks
Error message "Sum of cells are not equal to 1.0" until values in all four cells have been entered, not after a value is entered in a cell. (validate expression after all four cells have values or blank)

Scenarios:
c1 = .25, c2 is blank, c3 = .25, c4 = .50 TRUE no error message should displays
c1 = .25, c2 = .25, c3 = .25, c4 = .50 TRUE no error message should displays
c1 = .25, c2 = .30, c3 = .25, c4 = .50 FALSE error message should display after value is entered in c4
etc

What I don't want is for the error message box to display after .25 is entered in c1 because the user is still in the process of adding values.
Can this be done using Data Validation in Excel 2010?

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