Its possible through VBA ... but you'd have to have the macro running continuously checking to see if it was time to pop up the message ... which will probably impact on your PC's performance.
What else are you trying to achieve? If its just a pop up reminder, could you achieve the same by setting a recurring event in outlook? Or if you need it to be excel, could you have a workbook with a macro that triggers on opening, then use windows task scheduler to schedule it for every four hours?