That's simple.
A1:E1: DATES
A1:D1: Date1, Date2, Date3, Date4...
A2:D2: Val1, Val2, Val3, Val4...

$A$4=COUNT(A1:E1)-4 or
$A$4=COUNT(DATES)-4

Moving 4-Period Formula:
=SUM(OFFSET(A2,0,$A$4,1,4))

Remark: Insert New Column BEFORE last column which is E.
--
HTH
Tomek Polak, http://vba.blog.onet.pl