Yes, 78°C is average (with CPU usage hovering around the 25% mark) with a single copper finned heat sink on CPU (nothing for the other ICs) for the RPi4 4GB inside the official case. Leaving the cover open reduces the temperature by a few degrees.
I've used a micro-fan to lower the temperature to 64°C at similar CPU usage rates.
FWIW, an idle RPi3 B+ (headless mode) hovers between 46-49°C in my farm.
This topic is slightly contentious because some folks (not me!) understand the reasons rather well and also have above average knowledge on tweaking heat transfer coefficients for conduction and convection.