  We describe the structure of geometric quotients for proper locally
triangulable additve group actions on locally trivial A^3-bundles over a
noetherian normal base scheme X defined over a field of characteristic 0. In
the case where dim X=1, we show in particular that every such action is a
translation with geometric quotient isomorphic to the total space of a vector
bundle of rank 2 over X. As a consequence, every proper triangulable Ga-action
on the affine four space A^4 over a field of characteristic 0 is a translation
with geometric quotient isomorphic to A^3.
