  The Pais-Uhlenbeck(PU) oscillator is the simplest model with higher time
derivatives. Its properties were studied for a long time. In this paper, we
extend the 4th order free PU oscillator to a more non-trivial case, dubbed the
4th order time dependent PU oscillator, which has time dependent frequencies.
We show that this model cannot be decomposed into two harmonic oscillators in
contrast to the original PU oscillator. An interaction is added by the
coordinate transformation of Smilga.
