I had two Bitmain S4s being hosted offsite.  Recently, the PSU blew shortly after the 90 day warranty expired.  Apparently that's not uncommon with S4s.  Like others, Bitmain basically said "too bad, so sad" and refused to replace the junk PSU.  They then admitted their supplier only gives a 90 day warranty on their PSU.  In other words, the PSU is junk.