is this your first time working with cement? The biggest problem that I see will be finding a 4ft ball to use as the mold.

I agree-that's what i meant by a skeleton.

I understood the reference to skeleton. I was wondering if you had a "4ft" mold. Cover mold with mesh drywall tape. Apply slurry of cement to the outside of the tape. Continue to apply layers of cement until satisfied it is strong enough. Gravity will require this to be a multi day process. You may allow mold to remain covered, or before you complete remove ( most likely deflate the inflatable exercise ball) and then cover the hole.

is there really a 4ft inflatable ball?

any other ideas for the mold?

You could purchase a 40" inflatable Mega Ball for horses. It's $40 at Chewy...
or for about the same price, Amazon.com has a 6 foot (!) beach ball for $44. (There are also 4 foot and many other sizes.)
https://www.amazon.com/Happybuy-Giant-Beach-Diameter-Inflatable/dp/B07KX1JD7Z/ref=sr_1_2_sspa?keywords=giant+beach+ball&qid=1552814667&s=sporting-goods&sr=1-2-spons&psc=1

A metal fabricator could make your armature out of 1/4" steel rings. Large steel balls would look cool as well. I've seen this style made with galvanized straps.