How do I resolve 'Unable to join the network' issue during iPad 4 setup?

Start with basic WiFi troubleshooting - as detailed here:

If your iPhone, iPad, or iPod touch won’t connect to a Wi-Fi network - Apple Support

Recommended settings for Wi-Fi routers and access points - Apple Support

Consider that your Router, to which you are attempting to connect, may be configured to require support for security features that are not supported by older versions of iOS. If you have an older iPad and your router is configured for WPA3 or WPA3/WPA2 mixed authentication, you will need to change this to WPA2 PSK(AES).

Older models of iPad, such as your iPad4, do not support WPA3.

Have you already tried all the steps provided in this support article?:--> If your iPhone or iPad won't connect to a Wi-Fi network

Does it happen when you try it to connect it to a different wifi network too? (if it works, try rebooting your modem-router)

If the issue persists, I suggest that you contact Apple support directly:--> Official Apple Support

Regards

Giulio