You can use the labels of the expressions in your totaling expression. So Reg1+Reg2...+Reg4 will work if you set the labels of the expressions accordingly.
Column(1) is the first expression, Column(2) the second and so on. This is the order (top to bottom) of the expressions on the Expressions tab. The Promote and Demote button are used to change the order.
I had a similar problem when i stumbled upon this topic. Testing this I figured out following workaround which allows the use of dynamic names of expressions in a chart being used for other calculations in similar chart:
$(=chr(91) &'$(=XY)' & chr(93))
instead of XY insert the expression of the column label which should be used.