Z390 Pro4 and M.2

The proximity to the PCH heatsink could cause higher temps on that slot yes. That
said, it depends on how much hotter. As a general rule I try to keep M.2 drives
no hotter than 50c. If your temps are higher than that, see if you can improve
the airflow in that area.

Generally speaking, I have never seen any benefit in changing the stock PCH heatsinks.
You can find aftermarket heatsinks that have more mass and will keep the PCH a
bit cooler but most will impede expansion cards or foul on board components.
Space is limited and height is restricted by the PCIe slots.

About the only "upgrade" that may help (a little) is replacing the heatsink with
an identical sized one that has more fins, thus more surface area.

Also, bare in mind that if you are using a downdraft CPU cooler (fan blows down
onto the motherboard/CPU) this will be helping the temps of the top SSD.
In other words, some of the temp deficit is coming from the improved airflow
over the top slot. Airflow around the CPU socket is typically better than most
other areas in the case by design.

In my system my triple fan GPU pulls air directly over my lower m.2 slot and
keeps it nice and cool. To facilitate better thermal dissipation you need to
effect one or both of the following:
Surface area
Airflow

Airflow typically provides the best results.