I have noticed that the plasmashell process is using a lot of VIRT memory, the value si more than 250G; my system has 4GB of RAM and a 1GB swap partition.
Is this value normal?
Yes.
The VIRT column doesn’t tell anything about the actual memory usage.
See e.g. linux - What does Virtual memory size in top mean? - Server Fault
Thank you, I understand that virtual memory is just… virtual, but this value seems to be a lot high nonetheless; why does the desktop shell needs this?
Because it mmaps a lot of files maybe? (which means that the whole files show up in the process’s address space, i.e. VIRT value)
Some of it may also be pixmaps stored in the graphics card’s memory.
And so on.
In any case, this is irrelevant, don’t worry about that.
Your CPU has much more address space than 250 GiB 
Btw, have a look at the file /proc/kcore (which “contains” all the kernel’s addressable memory):
ls -lh /proc/kcore
-r-------- 1 root root 128T 22. Dez 19:35 /proc/kcore
Would you worry about this?
Thank you again for your explanation; I was trying to troubleshoot some heavy HD usage and very likely it is caused by high memory requirements of some applications that forces my (old) laptop to use a lot of swap. I was thinking that this could be related, but apparently it is not.
Thank you again.
The RES column in top should give a better view of the RAM usage of a process (and it will also display how much swap is currently in use at the top so you can see whether it is caused by memory requirements or just general IO).
Hint: press Shift+M to sort by RES.
OTOH, KDE’s systemmonitor (Ctrl+ESC) should do a good job too.