  We report Atacama Large Millimetre Array (ALMA) observations of four
high-redshift dusty star-forming galaxy candidates selected from far-Infrared
(FIR)/submm observations in the COSMOS field. We securely detect all galaxies
in the continuum and spectroscopically confirm them at z=3.62--5.85 using ALMA
3mm line scans, detecting multiple CO and/or [CI] transitions. This includes
the most distant dusty galaxy currently known in the COSMOS field, ID85001929
at z=5.847. These redshifts are lower than we had expected as these galaxies
have substantially colder dust temperatures (i.e., their SEDs peak at longer
rest frame wavelengths) than most literature sources at z>4. The observed cold
dust temperatures are best understood as evidence for optically thick dust
continuum in the FIR, rather than the result of low star formation efficiency
with rapid metal enrichment. We provide direct evidence that, given their cold
spectral energy distributions, CMB plays a significant role biasing their
observed Rayleigh-Jeans (RJ) slopes to unlikely steep values and, possibly,
reducing their CO fluxes by a factor of two. We recover standard RJ slopes when
the CMB contribution is taken into account. High resolution ALMA imaging shows
compact morphology and evidence for mergers. This work reveals a population of
cold dusty star-forming galaxies that were under-represented in current
surveys, and are even colder than typical Main Sequence galaxies at the same
redshift. High FIR dust optical depth might be a widespread feature of compact
starbursts at any redshift.
