Yeah, no. He wasn't (and still isn't) playing with complicated cards. He's 6 and a half now, and we played with Saboteur for the first time last night. He didn't understand what it did until I used it on him, and we walked through the text instruction-by-instruction. Then he thought it was awesome, especially when he hit one of my Provinces.
At age 4 the gist of his playing ability was that he learned how to count up his treasures, learned that you can't buy something for 5 when you only have 4, and memorized a small set of "good cards". With every kingdom, if he saw a new card he didn't know, he would ask me if it was a good card. Good cards became things that help buying Golds and Provinces... things like Laboratory, Council Room, Smithy, Bank, Cities, Library, and Goons. Workshop and Gardens weren't "good cards", but I taught him how they can dominate if they're together. Grandpa was impressed when Jack beat him for the first time that way.
I'd say that at that age his mind was essentially building a connection between the cards' pictures and their function... not really reading the text. There's a pretty big difference for a 4-year-old between playing IRL and playing at Making Fun. MF was a lot more fun for him, because he didn't have to execute the instructions on the card. He just clicked it and it happened... and he could basically remember how that kind of card would work next time.
He wasn't able to really create/improvise strategy. But he could remember and execute strategies I told him. He remembered things like that when playing Library, he should skip action cards when he had 0 actions left. He wouldn't buy Copper in general, but would if he had played a Goons. He knew that if he got all the Cities, they "went bananas".
MF was also more fun because the two of us could play cooperatively against Serf Bot. IRL, he had a real aversion to losing... made him not like the game. So I got stuck in a catch-22 of sorts... if I played dumb so that he would win and thus continue to like the game, he would mimic my purchases (e.g. 8 thieves). If I played well, he'd lose and not want to play again. Co-op vs Serf Bot was the answer.
The first time we did that, Witch was in the kingdom. I told him that Witch was a "good card" and we crushed it. We both had a great time and kept playing all the way until bedtime. However, the next day, the report from Mom was that Jack was running around the house all day saying "I'm a witch! I cursed you!" over and over. That's where I had the idea to make a chrome extension that removed those cards. Everybody happy.