Unless Kipketer knows something we don't know, i.e. there is a new undetectable PED doing the rounds, like the EPO of the late 90's, then there is no chance of any of these guys running 1:38 in 4 years. I don't expect anyone to even break 1:40.
The 800m has progressed less than most events in the last 30 years, despite it being one of the most widely raced distance, and this is because they have reached pretty much what is the limit. In real terms ( improvements in training, tracks, spikes, etc)it's hardly moved on at all since Coe's 1:41.7 in 81.