Is an iPod Touch good for a four year old?

If you can find one they can work, as could an iPad mini.

I think my sister also let my niece play with her old, offplan, phones too. Disable the cellular part and let them use the rest to play games or access other content.

You may wish to also investigate screentime and other limits to control a bit of what they can do (cause kids are almost scarily adept with technology)

Perhaps there may be a few online, some resellers had limited storage capacity models such as 32GB.

Higher models, such as 256GB; these can range (not incl. shipping/handling) way up to $800 or more.

Amazon has Apple iPod touch (6th & 7th generation) older models, for sale.

https://www.amazon.com/s?k=Apple+iPod+Touch+%286th+generation%29

https://www.amazon.com/s?k=Apple+iPod+Touch+%287th+generation%29

The price structure of their site seems to show lower price for Prime content.

The smaller capacity storage models, would use part of that storage for iOS

systems and apps usage; so the amount of available user-acquired content

appears to be much less than total; most system content is not removable.

resellers such as https://www.newegg.com presently show few iPod touch.

Made searches and saw "walmart" online Vendor stocks, much higher costs.

Perhaps an iPad mini, small model ~ from OWC -maybe Wi-Fi, may suit?

[Resellers such as OWC online have most things Mac, & are configurable.]

This year may be harder, because 'iPod touch' inventory low/Unavailable now.

There had been 'major sell-off' of this discontinued model series in May 2022.

[an iPod touch would've been a good one for younger persons when available]

While larger tablet size iPads, may be less difficult to find. ~Buyer beware.

This took a few hours time, some bookmarked sites; some from searches.