FIX BELOW:
After updating to iPadOS 18.4, many users—including those with iPad Air 4—have experienced AirPlay issues where their TV shows up in the list, but the connection either hangs or fails. This is likely due to a bug in iPadOS 18.4 that affects AirPlay’s ability to maintain connections, even when devices are on the same Wi-Fi network. Your iPhone (still on an earlier version of iOS) working fine confirms this is a software issue on the iPad. The Samsung Neo QLED TV is compatible with AirPlay, so it’s not the problem unless its firmware is out of date.
To fix it, first try fully powering off both the iPad and the TV (not just restarting) to clear any cached permissions. Make sure your TV’s firmware is fully up to date via its settings menu. You should also reset your iPad’s network settings (Settings > General > Transfer or Reset > Reset Network Settings) and reconnect to Wi-Fi. Make sure both devices are on the same Wi-Fi band (preferably 5 GHz), and temporarily disable any VPNs. If AirPlay still doesn’t work, the issue is most likely tied to the iPadOS update, and a fix may come in the next patch (like 18.4.1). Until then, using your iPhone for AirPlay may be the best temporary workaround.