When I try to download the Google Play application, it gives out "not enough space on the device", although I have 4GB of free memory. Why it happens?

And the answer to the question "why" was never heard.

Because the OS divides the internal memory (those 4GB of yours) into the system memory (applications and cache for them) and, in fact, non-system memory (music , pictures, everything else). Therefore, you need to remove unnecessary programs or transfer their cache and data to a USB flash drive. In the latest versions of android it seems to have been fixed, I don't know.

Google Play does this quite often ... even if you have a lot of free space on your internal memory. Personally, the following helped me: deleting Google Play app data and cache and Google Services

Delete the application cache. You can do this manually for each application, or download the application to remove everything together (if your smartphone is still capable of installing something). This usually happens if you are transferring a lot of data from your phone, for example, to an SD card.

Not enough system memory. Transfer already installed applications from the system to flash memory (4GB), this is done in the settings of each of them.

The Clean Master application is also recommended, but I do not know how it works and whether it helps.