Hi,
I use my new Pi4/4GB now since about a week with libreelec 9.2 build for Pi4. So far it works ok. There are still some small glitches to be ironed out, but generally the Pi4 works good and quite stable with libreelec/kodi. It's not as perfect as the Pi 3 builds yet, as it's still a quite new piece of hardware, but the developers at libreelec and kodi do great work. And for just watching movies or listen to music everything works good and is good for daily use.
I switched form a RPi3 B+ to a RPi4 4 / 4GB. Main improvement over Pi3 for me is the USB 3 and the Gigabit LAN, wich speeds up the data transfer significantly.
For Pi4 you don't have to buy the license keys, the CPU is by far strong enough to decode mpeg2 in software mode without problems.
If you decide to buy one, I recommend a good case for Pi4 (best metal with cooling option to avoid heat problems, like the Flirc case or similar) and I recommend explicitly the original power supply from Raspberry Foundation. Many problems and instabillities come from bad or weak power suplpies. As the Pi4 has microHDMI ports I also recommend an microHDMI/HDMI cable. Simple MicroHDMI adapters sometimes make problems. BTW you don't need to buy the 4GB or even the 2 GB version of the pi 4 for libreelec/kodi. But who knows what the future brings and the price difference isn't that big.
But if you want to save some money and don't need the 4K opportunities (and the Gigabit LAN and the USB 3 ports) , you still can be happy with the Pi 3. It runs rock solid with libreelec/kodi 18.5. For Pi 3I recommend buying the license keys for mpeg2 decoding...