Hi all ive just bought a fully working mac g4 (not a mini) with original keyboard, mouse and monitor. What i want to know is, is it possible to install amiga os 4.1 onto it ?
TIA
Nope!
For the Mini (which isn't what you have), there is a thing called "Moana", by which I believe you could bring up OS4.0 (not 4.1.x though) to at least a running state (although I don't think there are driver support for all HW controllers?). I have downloaded it myself, but never bothered to try it out...
For other kinds of Mac, you are completely out of luck.

Good news is that you could try out MorphOS (since beyond Pegasos 1 & 2 and Efika, MorphOS also supports eMac's (really cheap), Mac mini G4's (really small and powerful), Power Mac G4's (expandable, cheap and/or powerful), and soon PowerBook (laptop!)

completely for free!
Edit: http://morphos-team.net/downloads.html So what Mac do you have?