Hi makumbi,
You can use the Nz function to return zero, a zero length string, or another specified value when a Variant is Null.
Syntax
Nz(variant [,valueifnull])
variant : Required. A variable of data type Variant.
valueifnull :Optional
So the Expression is like below.
=Nz([4], 0)+Nz([5], 0)+Nz([6], 0)+Nz([7], 0)+Nz([8], 0)+Nz([9], 0)