She was 4.5yo if you want to be picky. Some of the hikes were:

https://www.alltrails.com/trail/us/montana/grinnell-glacier-... (from where we started, this was 19km return plus climbing around at the top; I carried her on my shoulders for less than 1km)

https://www.alltrails.com/trail/us/idaho/sawtooth-lake-via-i... (15+ km and rained for part of it)

https://www.alltrails.com/trail/us/utah/chesler-park?u=m (16km)

In 2021 when my other two children were 6yo and 9yo, they did 40+ km in 24 hours. Hiked midday until 10pm and then 7am until midday the day after. They are just normal kids; I am not fit but enjoy hiking.

Here's that story: https://sarogaining.com.au/news/almertas-bluff/ https://www.instagram.com/p/CUUTaCcJfN8/?igshid=MTc4MmM1YmI2...

I've not personally seen a 4yo in the backcountry but a 6yo has climbed our local highest peak. 30km round trip with 1.5km of climb.

And how about this family:

https://thetrek.co/pacific-crest-trail/big-trail-big-family/

Their 4yo is to date the only 4yo to complete the AT. Now they have done the CDT at 6yo. These are both >3000km treks.