Most people know where I work. So this is what I see most of with respect to Yosemite.
1. The upgrade from one Version of Mac OS to another is major, back the michine up first, often see bad installs due to bad internet and corrupted download, or because that there was an underlying hard drive issue.
2. Yosemite should run fine with 4gb ram, the main issues I see with slow running machines are due to old failing hard drives, large amounts a data on the desktop (simply this is referenced in cached memory, and is accessed all time in the background, initially is put in RAM, but when you run lots of APPs it has to move cache onto Hard Drive) if you need big folders or files on the desk top create aliases for them, finally if you have put File Vault on, unless you really need a highly secure encryption of you data switch it off.
3. If you only have 4gb of ram and open loads off apps then yes it will slow down as more ram is used up, certain apps are more memory hungry than others, you can see this using activity monitor. This is the same for Mac OS and Windows.
