orange light in speedo!

[Gearbox]

I've just got myself my first CB750 a CB750 Custom 1983 and she runs lovely except.....

I change up fine with no light coming on in the speedo but when I get in top, once I go above 4000 revs, the orange light comes on and stays on. Is this light a 'change gear advisory light' and if so why does it come on in fifth at 4000 revs when I can't change up any further anyway? I can go to 5000 revs in 4th and no light. Confused!! help appreciated.

Gearbox

[Gearbox]

UPDATE - No replies yet but I've just found out myself that the orange light in KM/H clocks on imported bikes is standard in USA and Japanese bikes to warn of speeds above 55mph or 88KMH. It's to do with their speed limititations I'm told.

Happy now now I know it's not warning of anything else engine wise!

Gearbox