Using the first 50% of data collected for the Spitzer Large Area Survey with Hyper-Suprime-Cam (SPLASH) observations on the 1.8 deg$^2$ Cosmological Evolution Survey (COSMOS) we estimate the masses and star formation rates of 3398 $M_*>10^{10}M_odot $ star-forming galaxies at $4 < z < 6$ with a substantial population up to $M_* gtrsim 10^{11.5} M_odot$. We find that the strong correlation between stellar mass and star formation rate seen at lower redshift (the main sequence of star-forming galaxies) extends to $zsim6$. The observed relation and scatter is consistent with a continued increase in star formation rate at fixed mass in line with extrapolations from lower-redshift observations. It is difficult to explain this continued correlation, especially for the most massive systems, unless the most massive galaxies are forming stars near their Eddington-limited rate from their first collapse. Furthermore, we find no evidence for moderate quenching at higher masses, indicating quenching either has not occurred prior to $z sim 6$ or else occurs rapidly, so that few galaxies are visible in transition between star-forming and quenched.
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