If you bored with default window decoration of KDE 4 you can change it. On system settings you can also download window decoration theme. Here's the step.
1. open System settings.
2. open Workspace appearance.
3. open Window Decorations.
4. choose the window decorations for archlinux KDE4 that you like.
5. Then apply, if you want to get another window decorations theme, choose get new decorations. You will get list of window decorations theme for archlinux with KDE 4.. :)
6. Choose Install then it will be listed. Then apply. :)