E.T.
i would call and see how they modify the party for 4 year olds. If you plan to have any guests younger than 4, i would go with the gymnastics party.
I did the gymnastics party for my son when he turned 4 and he LOVED it. He went to a cooking party for his classmate who turned 5 and he LOVED it. so much so that he now takes a cooking class about once a month.
however, there's no way i would've done the cooking party for him at 4. the party he went to was totally hands on and all the kids work. they kneaded the dough, leveled off flour, mixed ingredients, spread sauce, spread toppings, the works. you need a decent attention span for that.
the gymnastics party would cater better if you're going to have guests younger than 4. IMO.