I now have found the preferences (hidden in help -> about Wolfram -> System Information -> Front End -> Open Preferences).

When I set 4 kernel manually in the submenu 'Parallel' /Local Kernels it seems to work 'as it should' - cpu-count is still 1, but Mma starts with 1 kernel only and when I run something with parallel-evaluation, 4 kernels are starting and cpu-usage gets up to 100%. Thats fine. And this setting seems to be stored permanent.

But it would be great if this getting changed/fixed in a next release yet.

This does not save the setting permanently and furthermore starting all kernels immediately. Its useful as a workaround, but not very good when you have to open many different projects or need to take care of Raspberrys very limited memory.

Best solution would be saving 'Processor Count = 4' permanent anyhow...or fixing its processor-detection. Is there a way to save the 'Processor Count'-setting maybe?

Try this

Wolfram Language 11.2.0 Engine for Linux ARM (32-bit)
Copyright 1988-2018 Wolfram Research, Inc.

In[1]:= Unprotect[$ProcessorCount]; $ProcessorCount = 4;

In[2]:= LaunchKernels[]

Out[2]= {KernelObject[1, local], KernelObject[2, local], KernelObject[3, local], KernelObject[4, local]}