timer that has only 4 on and 4 off settings
 

I have a timer that has only 4 on and 4 off settings. As a result, I can only water four times a day. I would like to transplant a tomato plant from soil to growrocks with bits of rockwool. Will placing chunks of rockwool around the central portion of the root system provide enough moisture to the plant until the next watering?

Rockwool does hold moisture longer than rocks but I don't know if just 4 waterings will be enough. The principle behind hydroponics is that you want to force feed your plants so they won't waste energy having to produce roots looking for food. You want all the energy going to produce fruit so you need to make sure you supply your plant roots with plenty of nutrient rich solution as often as possible.