Splitting hourly data into 1/4 hourly data

Explanation:

Formula:

=IF(B5="",C4+(B5="")*(MAX(B6:B9)-MAX(B2:B5))/4,B5)

this part of the formula

Formula:

(MAX(B6:B9)-MAX(B2:B5))/4

will take a step between two values. Notice that it always need to be every 4th cell (that's why table can not start before 4th row) and all other values must be blank. (This is very specific issue, it's important to tell if you have some other requirements).

Now, when you got a step (+0.45 in first part, -1.8 in second) you need to add it to the previous value (C4) unless B5 already has a value (B5="")*.
If it has a value IF(B5="", will result FALSE and you need to take that value ,B5) as a result.