As I learnt this time last year the 04-07's have a cassette gearbox which just slides out of the side of the engine (so no need to split) my 4th gear broke 2 teeth off so I would check before something similar happens
hi have the same problem but only in 6 gear the gearbox is sweet going throu all the gears up untill you go over about 80/90 mph then it will feel like the chain as jumped the sprocket for a split second but only in 6 gear then its fine. got a 04 gearbox comming so will get it swopped over
you can prob get away with changing just the 4 gear but you wont no untill you pull the box out and see
hi mate if you are in a rush the guy i got the gearbox off is selling parts of one you may be able to get the 4 gear of him cheap enough HONDA CBR1000 RR4/5 GEARBOX PARTS | eBay