  The existence of submillimeter-selected galaxies (SMGs) at redshifts z>4 has
recently been confirmed. Using simultaneously all the available data from UV to
radio we have modeled the spectral energy distributions of the six known
spectroscopically confirmed SMGs at z>4. We find that their star formation
rates (average ~2500 MSun yr^{-1}), stellar (~3.6x10^{11} MSun) and dust
(~6.7x10^{8} MSun) masses, extinction (A_V~2.2 mag), and gas-to-dust ratios
(~60) are within the ranges for 1.7<z<3.6 SMGs. Our analysis suggests that
infrared-to-radio luminosity ratios of SMGs do not change up to redshift ~5 and
are lower by a factor of ~2.1 than the value corresponding to the local
IR-radio correlation. However, we also find dissimilarities between z>4 and
lower-redshift SMGs. Those at z>4 tend to be among the most star-forming, least
massive and hottest (~60 K) SMGs and exhibit the highest fraction of stellar
mass formed in the ongoing starburst (~45%). This indicates that at z>4 we see
earlier stages of evolution of submillimeter-bright galaxies. Using the derived
properties for z>4 SMGs we investigate the origin of dust at epochs less than
1.5 Gyr after the big bang. This is significant to our understanding of the
evolution of the early universe. For three z>4 SMGs, asymptotic giant branch
stars could be the dominant dust producers. However, for the remaining three
only supernovae (SNe) are efficient and fast enough to be responsible for dust
production, though requiring a very high dust yield per SN (0.15-0.65 MSun).
The required dust yields are lower if a top-heavy initial mass function or
significant dust growth in the interstellar medium is assumed. We estimate
lower limits of the contribution of SMGs to the cosmic star formation and
stellar mass densities at z~4-5 to be ~4% and ~1%, respectively.
