Water flow and pressure in a flat

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I'm not a plumber but a couple of things come to mind that could be checked/ruled out 

1. There might be another stop tap outside for your apartment, potentially a water meter was added but the water supply wasn't turned on again fully.

2. There might be isolation valve on the pipe as it rises up to the 4th floor, this pipe will just be for your apartment. It might not be fully open.

3. Sometimes apartment owners get a device installed that turns off the water if it's been running so long that it must be a leak. If you have one of these it might be faulty 

4. You might have a cold water tank that there's something wrong with and maybe an electric pump to create pressure and that isn't working well anymore. 

For all of these, the managing agency may not know much about the them. Good luck