DDR4 is available today, DDR5 has not even reached the preliminary spec stage yet and won't be available to consumer for another 3-4 years.
So if you need memory for a PC, DDR4 is your only option.
Also, DDR4 is still catching up with high-end DDR3 in terms of clock frequencies and timings, which means the net gains are close to zero if we compare the best cases of both technologies. DDR4's main benefit is lower power for similar performance. I doubt DDR5 will be much of a different story.