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IMO the "kid-aimed" toys like the Megachangers and Cogchangers are horrid pieces of trash. They are thin pieces of plastic that break easily especially with a kid hamhanding it- I know some people will argue kid's toys don't usually last long anyway, but it's not like they're even cheap $5 like G1 bumblebee or whatever was back in the day, they're like $15 each.
Instead, if your kid can wait a bit for it to be sent from an online store or you are lucky enough to spot it in a store somewhere, I recommend the Legacy G1 Optimus- it's a recent modern remake of the original G1 Optimus toy. IMO the original Optimus would actually still be a decent toy if it released today with 0 changes, and this is basically that except with no diecast and a bit more articulation. And unlike the Missing Link which is crazy expensive this goes for $25-$30. Yeah a little pricey for a little kid's toy, but IMO it's hardy enough to stand up to play for the most part and it's not like those thin pieces of shit like the Cogchangers.
If you think that's still too much for a kid's toy (or the G1 toy look is too off from the cartoon), then a Core Class Optimus might be a good choice- $10, looks good, simple enough to transform. Choking hazard maybe, but most 4 year olds should be smart enough to know better.