Upgrade from Raspi3 to Raspi4

[vatolin]

I have purchased a Raspi 4. Now I want to replace my Raspi 3. The Raspi 3 is running my NextCloud.

Does it do the job when only I shut off the Raspi 3, then replace the system SD card from the Raspi3 into the Raspi4 and unplug the USB HDD from the Raspi 3 and plug it into the Raspi 4 and then boot the Raspi 4? Or is it necessary to configure the Raspi 4 from the scratch?

[HawaiianPi]

Or is it necessary to configure the Raspi 4 from the scratch?

If your older Pi3 is running fully up-to-date Raspbian Buster, then it should work.

If the Pi3 is on Stretch or Jessie then you should start over from scratch with a new install of Raspbian Buster. New models require new firmware and kernel support. Since the release of the Pi4 was so close to the release of Raspbian Buster, Pi4 support was added to that. So the Pi4 won't run on older versions of Raspbian and requires Raspbian Buster.