2. An artist will make observations for many reasons every day. For one, an artist may draw something from life. An artist may also critique another piece of artwork and need to observe. An artist often needs observing skills to be able to evaluate their own work as well. I would say that mostly for artists though, observing is about receiving inspiration.
3. Copying isn't always plagiarism. A gray area comes in between inspiration and copying. An artist might just take a small but from another piece to become inspired. Otherwise, trying to take credit for the work of someone else is copying.
4. Artistic style is the style of a particular artist or movement. This can be described from huge art movements. Artistic style can also be on a more personal level. Maybe you always smear your paint on your art? This to me could also be artistic style.