OK Rich, I told you what I did with my son, now I will share my story as a child.
I never had LIONEL or any other “train set”.
My father was a Christmas time model railroader. He set up a Christmas Garden every year, but is was more than your average loop of track around the tree or a LIONEL set on a 4x6 with a few houses.
It was HO, running on TruScale wood roadbed track, two complete loops, on a layout that was 5’ deep and 18’ long. Every Thanksgiving they moved all the furniture around to make room for this in the living room. Him and my mother built the craftsman kit buildings of the era, Suydam, etc. It stayed up until early January.
The rolling stock was mostly Varney and Athearn metal kits, Amercican Beauty passenger cars, Pennline GG1, Varney F3 set, Mantua Pacific and Mikado built from kits, etc. It had an Aristo trolley bus loop, lighted houses and street lamps, etc.
When we finially moved into a house with a basement, he set the layout up down there, perminantly - but this time with grades, plaster mountains, hidden staging - yes hidden staging - and more. I was about 8 years old.
By the time I was 10, it was officially mine. I was building Silver Streak wooden freight cars, buying Athearn Blue box kits, changing the track plan, hand spiking TruScale ready track, etc.
At age 14 I was working in the local hobby shop and doing repairs, belonged to two clubs and the NMRA, and was building benchwork for a bigger layout - with hand laid track, hard shell scenery, and scratch building freight cars and structures.
So, here is my take, 4 is a little young for HO, but only a little, they grow up fast. I am already “training” my 2 year old grandson to handle HO - he can put his HO Bachmann Thomas and Percy on the track just fine and work my Aristo throttles.
I put a loop of Bachmann On30 under the tree with some “Christmas houses”, he runs that just fine with an Aristo wireless throttle.