Can I use eSIM on iPad Air 4 to access Wi-Fi while traveling?

All models of iPad can access WiFi networks worldwide. You only require an active eSIM or physical DataSIM to access Cellular networks.

If you have a WiFi+Cellular model iPad, you can activate cellular service by installing a DataSIM that you will obtain from a Cellular Carrier of your choice, or for supported models, by activating a Cellular plan using the iPad's built-in eSIM:

Set up cellular data service on your Wi-Fi + Cellular model iPad - Apple Support

WiFi Only models do not have any Cellular capabilities - and hardware cannot be retrofitted. It is not possible to change or modify the internal hardware components of an iPad. 

Whilst a WiFi Only iPad cannot be transformed into a WiFi+Cellular model, you can use the hotspot feature of a mobile phone - or a Cellular Broadband Router (often known as a MiFi Router) - to access Cellular networks:

How to set up a Personal Hotspot on your iPhone or iPad – Apple Support

iPad Air 4th gen with cellular has either SIM and eSIM (A2324, A2072), or has SIM only (A2325).

iPad Air 4th gen with Wi-Fi (A2316) has no cellular hardware.

Settings > General > About > tap on Model Number to see the A number.

Cellular cannot be retrofit. eSIM cannot be retrofit.

Wi-Fi is not tied to SIM or eSIM use, and a SIM or eSIM is not required for Wi-Fi.

You're very welcome.