Hi Ian. Signal BR8 is a 2 aspect Red/Green Stop signal.It would normally be used on lightly trafficked lines (say those with a low line speed or trains only every 1 or 2 hours) or as a replacement for a Semaphore stop signal ( with a red/white arm). It would often be preceded by Signal BR7 ( A Caution or Distant signal) so when you were approaching a signalbox you might find a BR7 1400 yards from the box then BR8 700 yards from the box and finally another BR8 800 yards past the box.If the 2 BR8's were showing green the BR7 would also show green, indicating to the Driver that he had a clear run through at full line speed till the next signalbox area.
Signal BR4 is a 3 aspect Red/Yellow/Green signal which combines the functions of BR7 and BR8.It can tell the Driver to Stop, Slow Down (expecting the next signal to show Red) or proceed at full speed.It would be typically used on busier lines with trains every 20 minutes or a higher line speed.
Signal BR4 has a "calling on" signal (known as Cats Eyes) fitted to it, but in practice normal signals on a main line would not have this and it doesn't work anyway so it should be ignored.
Hope this helps.
No, I didn't realise that! I've found it now, but although it's very good, it doesn't answer the point that I want to find out about, namely:-