How do you know it's gonna come after April 15th? I guess nobody knows when you actually gonna get your OPT... (btw, those 3-4 months, whatever time they say in USCIS is bogus). Actually, when did it reach the USCIS? Did you get any receipt yet? If it comes after April 2, I am pretty sure you will fk up your "grace period", as far as I know.