As Microsoft have said:
Today, we are releasing Windows 10 Insider Preview Build 17134 (RS4) to Windows Insiders in the Fast ring. This build has no new features and includes the fixes from KB4100375 as well as some fixes for general reliability of the OS. As Build 17133 progressed through the rings, we discovered some reliability issues we wanted to fix. In certain cases, these reliability issues could have led to a higher percentage of (BSOD) on PCs for example.
UPDATE: Microsoft have now released 17134 to the Slow and Preview Release rings; and it's pretty clear they expect this build to form the upcoming (but much delayed) Spring Creators Update release.