My tyros 4 has developed a fault on turning the keyboard on the button lights shine brightly but then dim. When pressing the buttons they work but show blue only dim red lights.

First, I would perform a master system reset to determine if this is a software problem, but I kinda doubt it. If the system reset doesn't work, it is likely a hardware problem and will have to be resolved by an authorized Yamaha service center.

Good luck,

Gary

agreed..................
don't try it yourself unless you electronically professional..

Besides of Gary's suggestion, and If I don't remember it totally wrong, there was a video at web about some weak soldering issues about lamps / buttons.
But, I don't think it was as many as you mention, and not too sure if it was T4 or T5.
If I find the video (most likely at YT), I'll post the link.

Edit:
Found this, and I think it was the T5 I had in mind....
Hope it's for any help.
Video about open / tear down a T4, and soldering bad lamp connection inside T5.

https://www.youtube.com/watch?v=iaxE4i9x_S4

https://www.youtube.com/watch?v=hJPl_Kp06AY&t=88s

On booting in Test mode the first screen i am getting an error message swerr : 070B