The presence of the line-of-sight vectors between the receiver and satellites in the design matrix of the GNSS single point positioning model hampers an easy analytical description of the variance-covariance matrix of the unknown parameters. However for the non-redundant case with only four satellites we present a closed-form, inversion-free expression for the variance-covariance matrix of position and receiver clock. This closed-form expression is shown to be a sum of the products of certain vectors with their transposed counterparts. These vectors are a function of the cross products of the line-of-sight vectors, however in their differenced form. We will relate this to the known PDOP/GDOP concept and present numerical examples.