Apple vacations offers nonstop flight from MKE to the Dominican Republic, and other charter companies over nonstop flights to the Caribbean. I'd call a travel agent for help with more options.
Keep in mind that your limit of a 4 day get away is going to limit you quite a bit since there is no daily service to any beach destinations further south than Florida.
My suggestion, NW nonstop to CUN on their weekly nonstop, with a connection in MSP on the way back.