How To Water Yam Planted In Sac Bag Very Good - For Proper Germination?

Can we water yams planted in bags when it has not yet germinated?

You can, but don't over wet them. Allow rain to fall properly before planting anything, my opinion though. Yes it needs water to germinate. 

Every 4 days interval. Yes, they require a lot of water at this stage. Two litres of water every four days in the absence of rain.

You can wet, once in a day maybe in the evening, I do so with mine and they have started geminating.

If you have planted already then there is need you water it while waiting for rain or it may whither or dry up .

But at most cases, there is no need for that. You can let it to grow first or wait for the rain. In the absence of rain water every 3-4 days. But make sure your sac bags is perforated to avoid water-logging.