Obscured fraction of star formation as a function of stellar mass (f_obs − M_⋆ relation) of our UV selected sample. Blue and red points show individual FIR continuum detections at 4 <  z <  5 and at 5 <  z <  6, respectively. Triangles show 3σ upper limits for IR nondetections. Stacks are shown by blue (at z ∼ 4.5) and by red (at z ∼ 5.5) rectangles. The nondetection of the stack is indicated by a downward arrow. Gray points and lines show the observed relation at redshifts between z = 0 and z ∼ 2.5 (Whitaker et al. 2017). The solid line shows the constant f_obs − M_⋆ relation of z ∼ 2.5 to z ∼ 0 using a template from Dale & Helou (2002), and the dashed line shows the same using Béthermin et al. (2015) or Magdis et al. (2012) templates. At M_⋆ <  10¹⁰ M_⊙, our z ∼ 4.5 stacks are potentially consistent with the f_obs at z ∼ 2.5 to z ∼ 0 using Béthermin et al. (2015) or Magdis et al. (2012) templates. However, at M_⋆ >  10¹⁰ M_⊙, our stacks show decreasing f_obs from z ∼ 2.5 to z ∼ 5.5, suggesting a rapid evolution of dust obscured star formation activity in main-sequence galaxies at z >  4. However, we caution that UV-selected samples at z >  4 may be incomplete at very high masses (M_⋆ >  10^10.5 M_⊙) given the absence of deep rest-frame optical imaging, as as been shown recently by the detection of a significant population of UV-undetected, massive galaxies (e.g., Wang et al. 2019; Alcalde Pampliega et al. 2019).