When will we have LPDDR4x memory in MBs/MBPs?
The actual "bottleneck" here is the fact that your "theory" is incorrect. 

The Intel CPUs used do not support LPDDR4 memory at all. It won't work no matter what you do. They support only DDR4 and LPDDR3 RAM. Only the 10th gen Intel mobile processors support LPDDR4 - and the new MacBook Pro has a 9th gen CPU.

The 10th gen Intel mobile processors have been announced and some models have just recently launched - they support the new LPDDR4 RAM, which is almost the only new feature. However, they're not used by Apple yet. It is logical to assume that at some point, Apple will switch to newer CPUs that support newer types of memory - but this time around was not it.