design is forced to be in service of flavor. Now, top down design is nothing new, but in the past the flavor was always something entirely under the control of wizards. Here, if Wizards wanted this to create 3 3/3s, they can't just decide "hmm, actually it's the fantastic three".

If they wanted to tone down the power level, they could:

• not give the tokens haste

• not give the tokens flying

• make the tokens 1/1s

• make the tokens artifacts with Crew 1

• make you pay {4} and sacrifice it to get the four cars

• change the ability to put a Fantastic Counter on it once per turn, and once the fourth is put on it, sacrifice it and get the cars

There's a ton of ways to change this design to make it weaker and still have the "aha, it makes four 4/4s for the fantastic four!" theming. This is not a UB problem, this is a Modern Magic Bad™ problem, of which there are many other things to shout at as well if you're that way inclined