  We revisit the TrES-4 system parameters based on high-precision HARPS-N
radial-velocity measurements and new photometric light curves. A combined
spectroscopic and photometric analysis allows us to determine a spectroscopic
orbit with an amplitude $K=51\pm3$ m s$^{-1}$. The derived mass of TrES-4b is
found to be $M_{\rm p} = 0.49\pm0.04 \rm M_{Jup}$, significantly lower than
previously reported. Combined with the large radius ($R_{\rm p} =
1.84_{-0.09}^{+0.08} \rm R_{Jup}$) inferred from our analysis, TrES-4b becomes
the second-lowest density transiting hot Jupiter known. We discuss several
scenarios to explain the puzzling discrepancy in the mass of TrES-4b in the
context of the exotic class of highly inflated transiting giant planets.
