I used to do a SOG grow with 6 inch cubes placed right next to each other. I would veg the plants for 1 week. I watered once a day in an ebb and flow system.
4 inch cubes is a bit small, it can be done. I have flowered a plant in a 3 inch by 4 inch cube before.
It would be best to lay a coco mat or something underneath them.
Hand watering is going to be really hard. They are going to drink so much. I guess it can be done. Don't go on any vacations.
check out my grow in the link in my signature. It is a SOG in 5 inch buckets with rockwool cubes in the buckets. I water once a day. :thumbsup: