You are correct, Sony A7r4 it has more definition due to the extra 20mp. Is it notorious? Well...it´s not very notorious but it´s there.
Negative points of having extra MP? There are not! If you don´t need them, you don´t buy it. If you need them you buy extra space in external discs. No one that need extra MP will complain about harddrive space.
There are 2 things though that you need to be aware, when you photograph with high MP sensors you need to bem extra carefull with shutterspeed and imperfections because everything will become more notorious. It´s common people that are use to lower mp sensors to have problems with no so crisp images and that most of the times is due to incorrect shutter speed. Other thing is ISO performance. It´s not very different from 42mp sensor, but the 60mp sensor of the A7r4 doesn´t have so good iso performance than a7r3.
So, if the question is, "should you get a higher MP camera"? Well in most of the cases, yes! In some cases like sports or photojournalism, you may not need so much because it will slow down your workflow, but...even sports photographers may benefict with higher MP. In other photo types, better resolution means better photos, evenif you dont need to print big. so the extra MP will be very well received.
I believe the 45 mp from this EOS R5 camera is a very sweet spot! It´s not too much and delivers you what you need in resolution! If you need more than this...you mustgo for the medium format cameras or wait for the 75mp camera from Canon (if ever will come).