>Скажите, пожалуйста, чем отличаются shared-объекты, имеющие расширения .so и so.4. Например, я
>скомпилировал библиотеку и получил два файла (их размер совпадает).

You knows that filename extension is not so important on Unix platforms. So, you can create dynamic library with "*.txt" extension. (it is just lyrical digression). The main thing of the number at the end of the dynamic library's filename is to detect version and keep back compatibility with other versions of this library installed on your machine. For example, you can install two instances with different versions of the MySQL on a single server and you'll got some shared libraries, for example "libmysqlclient.so.4" and "libmysqlclient.so.5". Usually "libmysqlclient.so" is not a library, just a link to the higher version of the library. If it is not exists, create it manually.

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Thanks,
   Vladislav Lazarenko.