From what I know, once you file H1-B in April and and receive receipt from USCIS, you will be fine. The USCIS receipt date has to be prior to your OPT expiration date. After that you will still be status even if you OPT will expire. However, you cannot travel outside US or start working till you get decision regarding your H1-B. If you get H1-B approved, party on man. If not, they will give you 30 days to leave the US.

It's always better to talk with a lawyer though. Good luck.