  We investigate the validity of Taylor's Hypothesis (TH) in the analysis of
Alfv\'enic fluctuations of velocity and magnetic fields in solar wind streams
measured by Parker Solar Probe (PSP)~during the first four encounters. We use
PSP velocity and magnetic field measurements from 24 h intervals selected from
each of the first four encounters. The applicability of TH is investigated by
measuring the parameter $\epsilon=\delta u_0/\sqrt{2}V_\perp$, which quantifies
the ratio between the typical speed of large-scale fluctuations, $\delta u_0$,
and the local perpendicular PSP speed in the solar wind frame, $V_\perp$. TH is
expected to be applicable for $\epsilon\lesssim0.5$ when PSP is moving nearly
perpendicular to the local magnetic field in the plasma frame, irrespective of
the Alfv\'en Mach number $M_{\rm A}=V_{\rm SW}/V_{\rm A}$, where $V_{\rm SW}$
and $V_{\rm A}$ are the local solar wind and Alfv\'en speed, respectively. For
the four selected solar wind intervals we find that between 10% to 60% of the
time the parameter $\epsilon$ is below 0.2 when the sampling angle (between the
spacecraft velocity in the plasma frame and the local magnetic field) is
greater than $30^\circ$. For angles above $30^\circ$, the sampling direction is
sufficiently oblique to allow one to reconstruct the reduced energy spectrum
$E(k_\perp)$ of magnetic fluctuations from its measured frequency spectra. The
spectral indices determined from power-law fits of the measured frequency
spectrum accurately represent the spectral indices associated with the
underlying spatial spectrum of turbulent fluctuations in the plasma frame.
Aside from a frequency broadening due to large-scale sweeping that requires
careful consideration, the spatial spectrum can be recovered to obtain the
distribution of fluctuation's energy among scales in the plasma frame.
