I would avoid 2x4s, those are for building houses, not furniture. If you want them to be super sturdy (workbench top) I would laminate 3/4 plywood to make inch and a half plywood to make the boxes so you have inch and a half uprights and top. Then put doors on it. Other than that "make them heavy duty" advice, there are a ton of cabinet making resources on line and at your local hardware store.
Off the top of my head I'd highly recommend the book "How to make Kitchen Cabinets" published by American Woodworker. That's one of my favorites.