Does "DDR4L" memory exist?
Solution 1:
According to an unreliable site (Wikipedia) and a most reliable site (JEDEC):
Regular DDR4 runs at approximately 1.2V – not 1.5V – so it is already even lower than DDR3L.
DDR4L (running at even lower 1.05V) allegedly was in the plans at least until 2014, but so far no such specification has been published by JEDEC, so officially it does exist at this moment.
LPDDR4 and LPDDR4X (Low-power DDR4) does exist (1.12V and 0.6V respectively), but it seems to be meant as soldered-in memory for embedded systems, not SODIMMs.