You will not be able to return to the US if you travel to Mexico since you do not have a valid F-1 visa stamp in your passport.  You will have to obtain a valid F-1 visa first before being able to return to the US and return to your MBA program.  It is unlikely that the US Consulate in Mexico will entertain an application for a F-1 visa there if you are only there for a brief 4 day vacation but this is possible if you find out in advance whether they will accomodate you.  If not, however, know that you will need to travel to your home country to apply for the visa directly from Mexico since you have no way of getting back into the US.