[Cisco2kid]

Just assembled my new PI4b and powered up.  Just inserted the micro card from PI3.  No signal.  Is my new pi DOA or do I need to redo the microcard for it?
Have the new HDMI cable into the back of the home theatre amp as before.  All else shouild be the same.  I suppose it could be a bad HDMI cable?  Thanks for any tips.

[Cisco2kid]

Ok, here myself.... quick answer... yes, it needs the new arm image for Pi4..... but there is another big problem and now I am finding on the internet that the Pi4s are really buggy.  I have to ask my new question in another post, thought I hope my answer here helps someone else perhaps.

[wodkab]

I had the same problem ... none of my RPi3 cards are booting. I had to use Noobs to get Libreelec running on my RPi4.

[DarrenHill]

The Pi4 needs new and current firmware, which presumably your old Pi3b+ card didn't have.

I have seen some people messing around with dist-upgrade and such like to try and get it to work, but the simplest way is to start fresh with a new Buster install and copy over what you may need to bring with you.

[mcelliott]

 now I am finding on the internet that the Pi4s are really buggy.

The software support is not quite there yet, but I don't find anything buggy about my Pi4. I am using the latest LibreElec beta, not messing with Noobs or Buster.

[Klojum]

I had to use Noobs to get Libreelec running on my RPi4.

Then you are doing something wrong. LibreELEC can certainly do without Noobs.

[Leatherface]

I did flash RPi4 update on my RPi3 with .nocompat file added then moved SD card to my new RPi4.
Worked fine for me.