What is the source of your info? When you say a difference of 3-4 degrees, is that lower or higher than air temp?
The VPD information and charts I've seen indicate leaf temp 1-2C lower than air temp and that's what people here have measured, including myself.
I hope this link works, I seem to have a browser hijacker and can't get to it.
Vapor Pressure Deficit - The Hidden Force on your Plants | Just For Growers
My source of info is people who grow under led.
And the temperature is lower than air temperature of course.
With HPS it is already 1 or 2 degrees lower, and that is with a light source that gives a lot of radiant heat.
Radiant heat, heats up surfaces (like leaves) not the air directly.
LED does not have radiant heat, so the only heat coming from LED is convection heat. It will heat up the air.
Envetually also the leaves, but at a much slower pace.
The charts you see on cannabis related sites, are mainly looking at HPS as the source of light.
With this link you can adjust the difference between ambient and leaves. You can slide the bar.
It also gives more realistic numbers.
Some of the charts you see online have no scientific basis.