Automorphisms of Real 4 Dimensional Lie Algebras and the Invariant   Characterization of Homogeneous 4-Spaces
  The automorphisms of all 4-dimensional, real Lie Algebras are presented in a comprehensive way. Their action on the space of $4\times 4$, real, symmetric and positive definite, matrices, defines equivalence classes which are used for the invariant characterization of the 4-dimensional homogeneous spaces which possess an invariant basis. 
T. Christodoulakis, G.O. Papadopoulos, A. Dimakis