Hi there, Jennifer!
Many of the restrictions in place at Walt Disney World are in place for the protection of the Guests, the Cast Members, and the property. That's why, of course, you'll find certain rides "off-limits" based on height, for example.
The
Star Wars Jedi Training Academy at
Disney's Hollywood Studios is, as you point out, geared toward children 4 and up. However, if there are no safety rules governing this rule, and if they are permitted, sometimes Cast Members can exercise a little bit of personal judgement on a case-by-case basis.
It is possible that child-testing of this interaction demonstrated that children under four are simply not ready to wait their turns to battle Darth Vader (standing relatively in one place) and/or participate fully (i.e. following the fast-paced directions given by the show host). You and the Cast Member at sign-up can determine this together specifically for your little ones.
Consider checking out some fan videos of the experience on sites like YouTube. That will give you an idea of what each child will need to do, and you can decide if, in fact, it is right for your twins.
Happy planning!
See you later, celebrator,
-Deborah B,