OK Nid! It takes little to get me started and lots more to get me to stop. You can tell this tale to audio folks at a bar sometime. It is worth a free beer.
HOW +4 BECAME OPERATING LEVEL
Many audio standards can be traced back to early telephone systems which used balanced lines and 600 ohm circuits. You don't need electronics for telephony, but sooner or later line losses force you to add amplification which makes you need to know gain which makes you need a meter.
Line level (the word line comes from telephone line) was around 1mW as they set up the system - thus 0dBm is defined as 1mW in 600 ohms. To meter this they needed to measure things around that level with an AC voltmeter that was fast enough to respond to speech but not so fast as to measure the pops and clicks that were pervasive on the system. The VU meter was designed to respond to the ballistics of speech, and speech only, so you could measure signal loudness on the lines. Units of Volume, you might call it.
Unfortunately, the little motors that threw around the needle's mass required a bit of current - enough to load down the line a slight amount. {An old fashioned passive voltmeter like a Simpson is only accurate for 60Hz AC and the needle swings slowly, but there is extremely little current drawn which is essential for an accurate voltmeter}. If you added 3600 ohms in series with with the VU meter, the signal level would not perceptibly change as you switched it on and off a line. Of course this resistance desensitizes the meter - by exactly 4dB! Thus, +4dB became the standard for line level!
Epilogue:
Remember that this meter is good for speech. Transient things will get by it. When you add this to the fact that analog tape EQ makes it so tape saturates easier at high frequencies than low ones, you will understand why savvy recording engineers will bury a kick drum channel's meter and barely tickle the meter on hi-hat or direct Fender Rhodes. You gotta know how to work AROUND the meters!
Now you know why British console manufacturers gag when you ask them to remove their nice PPM's (which REQUIRE complex electronics to measure instantaneous peak amplitudes and hold it long enough for the human eye to see) in order to put in VU's which miss musical signals and will cause distortion if not buffered.