I understand that keyboarding without tears has units set by age group, so I think that program could be a good option. If there isn't a unit for his age, then he simply is too young.
Understand that there is a lot of developmental readiness involved in learning how to keyboard, and 4 years old isn't going to have that. Your son should be able to learn some simple moussing skills and a few basic keys, but it will be a long time before he is truly keyboard ready. The OT my son had did not deem him ready to learn to type (as in really type) until he was in 6th grade. Before that he played around with keyboards and introductory lessons, but didn't do much more.
I wouldn't push him on it. When there are age appropriate programs he wants to use that can only be accessed by keyboard and mouse, he will have the motivation to figure out what he needs to.
My son lives on his computer, btw and is an excellent touch typist. But most of that progress started from middle school forward. Before that, it was a little moussing and the occasional hunt and peck typing and that was it.
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Mom to an amazing young adult AS son, plus an also amazing non-AS daughter. Most likely part of the "Broader Autism Phenotype" (some traits).