Assuming List A is in Column A data starting from row 2 & you only have numerical values & also only 1 entry per row you could easily use the sum formula:

=sum(A2:D2)

if you have multiple entries per row it will total these, and if you have non numerical data it will not work.

More complex you could use the following for all scenario's:

=if(A2<>"",A2,if(B2<>"",B2,if(C2<>"",C2,D2)))

Regards
Mark