Excel sum every nth row different sheet

I have two excel sheets: 1 for expenses that is monthly and another that is a yearly summer of expenses by expense type.

For the yearly expense sheet, I would like to sum every 12 months an put it in a cell, then the cell next to it is the next 12 months, and so on.

So to give an idea of how my sheet is set up:

Sheet titled 'Operating Expense' has cells B4:M4 that are Jan-Dec of that year for the monthly expenses.

On The sheet titled "sources & uses of funds", i would like to have call D28 be the sum of B4:M4, and then cell D29 the sum of N4:Y4 and so on until I stop dragging the formula across.

I tried the Sum(offset()) function but couldn't seem to figure it out.

Thankyou!