If you can get R13 in the 3.5 inch spacing of the 2x4 walls, and the styrofoam is the typical R5.5 for one inch, it sounds close to the R19 he is telling you. Actually, with the boundary layer effect, plus the insulating value you get from whatever siding you put on, it will probably put you over R20 total. Another thing to remember is that the styrofoam goes over the 2x4s, not just between them. This will give you better total insulating value than 2x6 walls with R19 fiberglass only between the studs. I lived for many years in a house with 2x6 walls, 1 inch styrofoam in the outside, and 5/8 inch rough sawn siding over the styrofoam. The walls were solid, propably because of the plywood siding. If you are going to put vinyl siding directly over the styrofoam, then I would be concerned with how solid the walls would be.