What you are talking about is a small 3-brane. One can formulate such a theory, but unlike the string theory (1-brane), the quantum theory contains UV divergences that cannot be eliminated, at least not perturbatively. In fact, UV divergences can be (perturbatively) eliminated only for objects extended in 1 dimension (not 0, not 2, ...), which is related to the fact that the conformal group in 2 dimensions (the other dimension is due to the extension in time) is much reacher than in other numbers of dimensions.