H1B - H4 - H1B

I am working in US on a valid H1B visa (validity till 2018).

I am planning quit my job by 31st Jan and convert H1B to H4 by COS and then find a new job to work on H1B again. My questions:

  1. What could be the reasons my H1 to H4 COS be denied? And what is the best time to apply for COS.

  2. If COS is denied, and i am staying in the US, will my stay still be legal?

  3. After getting converted to H4 status, and say i get a job in the month of March, what would be the process of conversion to H1B status?

  4. Will my previously approved petition be valid still and is it taken into account?

  5. Would a new H1 petition need to be filed again in April with the regular cap?And in that case, would i need to go for H1b stamping again to my homecountry or is there a way to do COS?

Please suggest. Thanks!