Mr.Online,
The statement you refer to "But it also says i should change AC fluid every 4th year..." may sound crazy on the surface.... but depending on the type of compressor you have this is not out of line... they may only be referring to the special cold flowing oil INSIDE your compressor sump... it does get some contamination over time and should be changed... same principle as changing the oil inside the vacuum pump which is used to evacuate your system.
I suspect you do NOT have the Delco compressor most of us have.. but either the York or a Nippon , or GM .... those have sumps which contain oil..