You can create 2 DateTime variables with the addhours before the retrieve. Then use the variables in your xpath.
For completeness, this can be done purely in XPath without using extra variables. This is especially good to know if the XPath is in the context of a page where variables can not be used.
[Start >= '[%CurrentDateTime%] + 24 * [%HourLength%]' and Start <= '[%CurrentDateTime%] + 28 * [%HourLength%]'] [Active] [ReminderSend = empty]
For performance reasons, it is best to use the most constraining expression first.
As per Maarten’s answer, perfect, thank you.