What's your exact expression? Column 5 should be similar to:
I suspect than one of your records somewhere in your table has a null value. If you want to treat a null as zero, you'll need to build four calculated fields, and then use those in your summary.
Each of your calculated fields will need a formula similar to:
Monarch won't add up null values. 2 + 1/0 = null. So a null anywhere will give you a null total, not whatever the other three add up to when one is blank (which is probably "null").