I think most parents would describe their four-year-olds this way. It is very common for children this age to want to eat the same food all the time and to have very specific likes and dislikes. They also tend to change their pattern of likes and dislikes without apparent reason. This summer he loves peanut butter and jelly, in the fall it may be only bologna and cheese. Because of this, it is reasonable to keep trying new or different foods on a recurring basis.
In spite of this pickiness, most children actually get all the calories, vitamins and minerals that they need. Unless your child totally excludes one particular food group, for example meat or dairy, it is unlikely that he actually needs a supplemental vitamin. Assuming he is an otherwise healthy child, taking the vitamin won't hurt him, and it may help you feel less anxious about his eating patterns. He is old enough to take a children's chewable vitamin like "Flintstones", though a generic equivalent would be fine and less expensive.