I'm having difficulty creating a formula that will automatically add the
figures in cell C8 on the current sheet, plus the next 3 sheets. I need cell
C8 to remain static, but the current sheet and next 3 sheets to change
according to whichever sheet I use that formula in. For example,

A formula where it sums up the figures from C8 on the current sheet, being
sheet1, plus the next 3 sheets, being sheet2, sheet3 and sheet 4...then

The same formula, if used on sheet 12, will sum up the figures from cell C8
on the current sheet, being sheet12, plus the next 3 sheets, being sheet13,
sheet 14 and sheet 15...and so on.

Note: Each of my sheets have different names according to the week, i.e.
Jan 4-8, Jan 11-15, Jan 18-22, Jan, 25-29, etc.