A detailed discussion is given of the analysis of recent data to obtain improved upper bounds on the couplings $|{U}_{e4}{|}^{2}$ and $|{U}_{\ensuremath{\mu}4}{|}^{2}$ for a mainly sterile neutrino mass eigenstate ${\ensuremath{\nu}}_{4}$. Using the excellent agreement among the $\mathcal{F}t$ values (products of kinematic rate factors times half-lives with radiative corrections included) for superallowed nuclear beta decays, an improved upper limit is derived for emission of a ${\ensuremath{\nu}}_{4}$. The agreement of the ratios of branching ratios ${R}_{e/\ensuremath{\mu}}^{(\ensuremath{\pi})}=\mathrm{BR}({\ensuremath{\pi}}^{+}\ensuremath{\rightarrow}{e}^{+}{\ensuremath{\nu}}_{e})/\mathrm{BR}({\ensuremath{\pi}}^{+}\ensuremath{\rightarrow}{\ensuremath{\mu}}^{+}{\ensuremath{\nu}}_{\ensuremath{\mu}})$, ${R}_{e/\ensuremath{\mu}}^{(K)}$, ${R}_{e/\ensuremath{\tau}}^{({D}_{s})}$, ${R}_{\ensuremath{\mu}/\ensuremath{\tau}}^{({D}_{s})}$, and ${R}_{e/\ensuremath{\tau}}^{(D)}$, and the branching ratios $\mathrm{BR}({B}^{+}\ensuremath{\rightarrow}{e}^{+}{\ensuremath{\nu}}_{e})$ and $\mathrm{BR}({B}^{+}\ensuremath{\rightarrow}{\ensuremath{\mu}}^{+}{\ensuremath{\nu}}_{\ensuremath{\mu}})$ decays with predictions of the Standard Model is utilized to derive new constraints on ${\ensuremath{\nu}}_{4}$ emission covering the ${\ensuremath{\nu}}_{4}$ mass range from MeV to GeV. We also discuss constraints from peak search experiments probing for emission of a ${\ensuremath{\nu}}_{4}$ via lepton mixing, as well as constraints from pion beta decay, CKM unitarity, $\ensuremath{\mu}$ decay, leptonic $\ensuremath{\tau}$ decay, and other experimental inputs.