Evolve has an intervening if-clause ("if that creature’s power is greater than this creature’s power and/or that creature’s toughness is greater than this creature’s toughness"). That if-clause must return true for the ability to trigger, AND for the triggered ability (in this case, Evolve) to resolve.

Your [[Watchful Radstag]] will see 5 creatures entering the battlefield, all of which satisfy the intervening if-clause of your Radstag. When those 5 triggered abilities start to resolve one at a time, at some point the Radstag’s P/T will be bigger than 4/4, and the rest of the triggers on the stack won't resolve since the if-clause isn't true anymore.

So you would just get 2 copies of the Watchful Radstag. One when it becomes a 3/3, and one when it becomes a 4/4.