Flushing Cache in SE4 is not a difficult task
Usually it is required after language change, moving site to another host or to check updated result of template
You can normally flush the catch via the admin panel but this does not always get rid of all the temporary files used.
Here are step:
1. login to ftp root of your se4 site
2. rename current temporary folder to temporary1
3. Extract some clean install package on your pc and upload its TEMPORARY folder again to your SE4 root
4. chmod 777 recursive to this newly uploaded temporary folder
And you are done!