  $^4$He confined in nanoporous media is a model Bose system that exhibits
quantum phase transition (QPT) by varying pressure. We have precisely
determined the critical exponent of the superfluid density of $^4$He in porous
Gelsil glasses with pore size of 3.0 nm using the Helmholtz resonator
technique. The critical exponent $\zeta$ of the superfluid density was found to
be 1.0 $\pm$ 0.1 for the pressure range 0.1 < P < 2.4 MPa. This value provides
decisive evidence that the finite-temperature superfluid transition belongs to
the four-dimensional (4D) XY universality class, in contrast to the classical
3D XY one in bulk liquid 4He, in which $\zeta$ = 0.67. The quantum critical
behavior at a finite temperature is understood by strong phase fluctuation in
local Bose-Einstein condensates above the superfluid transition temperature.
$^4$He in nanoporous media is a unique example in which quantum criticality
emerges not only at 0 K but at finite temperatures.
