Risk of visiting Mexico for 1 day on H4 visa when H1B approved

I am from India and on H4 visa valid till 2016 and my COS is approved and H1B is valid from Oct 1st 2015 onwards. I am planning to visit Mexico and will be back to US same day.

Q1. If visit is before October 1st

Q2. If visit is after October 1st

I want to know the Risk of visiting Mexico for 1 day on H4 visa when H1B is approved but I have not started working, will it require me to have the visa stamped with H1B?

As I want to work from October 1st 2015.

Usually it not advisable to travel with a pending CoS. I’d suggest start working on Oct 1st, then travel. For a short trip to Mexico you don’t even need a stamp to come back. You just come back using Automatic Revalidation. Just make sure that when you re-enter the country the CBP puts H1 on your I-94. Some of the CBP officers are not familiar with this themselves so find the Automatic Revalidation rule, print it and take it with you.

Thanks for the reply.
1- Here point is as H1B petition is approved, I believe COS is not pending, its approved?
2- If I travel before October 1, 2015 am I supposed to have I94 status at entry with H1B or H4 as before October 1, 2015 my H1b is not valid, As per approved petition the H1B status is valid with future start date of October 1, 2015.
3- Where can I find the automatic revalidation rule for this scenario and what other related documents I may need to carry befere travelling october 1. I can’t get H1B stamped as I will return to US before October 1,2015.