Basically, ER4 = ER3 + 75 ohms
And back in the days, it was the same
ER4S = ER4P + 75 ohms
And @csglinux proved this as well, and the update is, Ety implanted the 75 ohm resistor inside the driver housing.
The ER4SR is only 45 ohms though, so wouldn't the ER3SR with a 75 ohm adapter be harder to drive?