K4/K5 are the same bike in the sense the design, the guy who was asking you probably wasn't sure what he was asking himself. People get confused, lets say an Audi S4, someone will ask you what body style is it? then you can reply B4 or B5 < which dosen't mean its 2004 or 2005 the models could be some other years, because those are actual B=body codes for the make, or like VW use... mk1 mk2 mk3 mk=make.....Suzuki bikes use "K" which stands for "YEAR" like Y2"K"=Year 2000< "K", so K4 or K5 = 2004 or 2005, therefore in short form suzuki recognizes their body codes by year K. Hope this makes sense. So when someone asks you what bike is it, you can either say 2005 gsxr 600 or if you want to say it quick K5 gixxer 600. if you had a 2004 you would say K4, the guy should have understood you have a K5 when you said your bikes 2005. He obviously was a noob all things motor.