  Asteroid 2012 TC4 is a small ($\sim$10 m) near-Earth object that was observed
during its Earth close approaches in 2012 and 2017. Earlier analyses of light
curves revealed its excited rotation state. We collected all available
photometric data from the two apparitions to reconstruct its rotation state and
convex shape model. We show that light curves from 2012 and 2017 cannot be
fitted with a single set of model parameters -- the rotation and precession
periods are significantly different for these two data sets and they must have
changed between or during the two apparitions. Nevertheless, we could fit all
light curves with a dynamically self-consistent model assuming that the spin
states of 2012 TC4 in 2012 and 2017 were different. To interpret our results,
we developed a numerical model of its spin evolution in which we included two
potentially relevant perturbations: (i) gravitational torque due to the Sun and
Earth, and (ii) radiation torque known as the
Yarkovsky-O'Keefe-Radzievskii-Paddack (YORP) effect. Despite our model
simplicity, we found that the role of gravitational torques is negligible.
Instead, we argue that the observed change of its spin state may be plausibly
explained as a result of the YORP torque. To strengthen this interpretation we
verify that (i) the internal energy dissipation due to material inelasticity,
and (ii) an impact with a sufficiently large interplanetary particle are both
highly unlikely causes its observed spin state change. If true, this is the
first case when the YORP effect has been detected for a tumbling body.
