We present galaxy stellar mass functions (GSMFs) at \(z=\) 4-8 from a rest-frame ultraviolet (UV) selected sample of \(\sim\)4,500 galaxies, found via photometric redshifts over an area of \(\sim\)280 arcmin\(^2\) in the CANDELS/GOODS fields and the Hubble Ultra Deep Field. The deepest Spitzer/IRAC data yet-to-date from the Spitzer-CANDELS (26.5 mag, 3\(\sigma\)) and the IRAC Ultra Deep Field 2010 (26.4-27.1 mag, 3\(\sigma\)) surveys allow us to place robust constraints on the low-mass-end slope of the GSMFs, while the relatively large volume provides a better constraint at higher masses compared to previous space-based studies. Supplemented by a stacking analysis, we find a linear correlation between the rest-frame UV absolute magnitude at 1500\AA\ (\(M_{\rm UV}\)) and logarithmic stellar mass (\(\log M_*\)). We use simulations to validate our method of measuring the slope of the \(\log M_*\)-\(M_{\rm UV}\) relation, finding that the bias is minimized with a hybrid technique combining photometry of individual bright galaxies with stacked photometry for faint galaxies. The resultant measured slopes do not significantly evolve over \(z=\) 4-8, while the normalization of the trend exhibits a weak evolution towards lower masses at higher redshift for galaxies at fixed \(M_{\rm UV}\). We combine the \(\log M_*\)-\(M_{\rm UV}\) distribution with observed rest-frame UV luminosity functions at each redshift to derive the GSMFs. While we see no evidence of an evolution in the characteristic mass \(M^*\), we find that the low-mass-end slope becomes steeper with increasing redshift from \(\alpha=-1.53^{+0.07}_{-0.06}\) at \(z=4\) to \(\alpha=-2.45^{+0.34}_{-0.29}\) at \(z=8\). The inferred stellar mass density, when integrated over \(M_*=10^8\)-\(10^{13} M_{\odot}\), increases by a factor of \(13^{+35}_{-9}\) between \(z=7\) and \(z=4\) and is in good agreement with the time integral of the cosmic star-formation rate density.