Several points to add:
#1 - It should be stressed that the exhibition board at Zhuhai did not mention that the DF-100 has a cruise speed of Mach 4, only that the DF-100 is able to reach speeds of Mach 4 (速度高达4马赫). So it is probable that the DF-100 has a lower average cruise speed, and that Mach 4 is achieved during its boost and/or terminal flight stage.
#2 - DF-100 entered service with the PLARF too little, too late. While having large similar starting points in time as the other hypersonic-speed missiles, the DF-100 project has suffered terribly from lengthy delays in its development. @万年炎帝 also stated at the same time as the DF-100 basically completing its acceptance trials, 祝学军院士's DF-17 and 朱广生院士's DF-26A are already in serial/mass-production and deployment stages - Something which Ayi agrees.
#3 - One arguably more important factor in the PLARF's decision to induct only small numbers of DF-100 lies with the changing nature of anti-ship/land-attack missions of today and going forward.
DF-100's presence back in the 1990s, 2000s or even early-2010s would've been monumental. However, with the advent of high/very-high-altitude hypersonic missiles on one end and low-altitude, subsonic VLO missiles on the other end - Supersonic high-altitude cruise missiles like the DF-100 increasingly becomes an oddity.
Given that DF-100 travels in supersonic speeds throughout pretty much all of its journey, the missile must stay at high altitudes to reduce drag in order to retain its huge strike ranges. Yet, such missiles cannot travel too high either, given that it is air-breathing (meaning thinner atmosphere). The speed at which the missile travels at also means a rather large IR signature (due to greater amount of air friction), which diminishes the purpose and effort at making such designs (V)LO-shaped.
Many of the anti-air missile systems today (HQ-9B, SM-6, PAC-3 MSE etc) are already capable of travelling as fast as, if not faster than the many of the supersonic missiles of today, while also getting able to intercept missiles at higher altitudes. DF-100's induction today means it's pretty much stuck in an environment that isn't exactly the friendliest to it, especially when compared to either of the two ends of the spectrum.
(The same negatives apply with the likes of YJ-12, although YJ-12 got lucky that it was inducted into active service much earlier than the DF-100.)
Therefore, it is understandable (if not natural) for the PLA to relegate the DF-100 for second-line strike duties, given the much superior strike options that are already widely available at the PLA's disposal today and into the future.