What is this person’s definition of “creep feed”? Some people define it as allowing free choice access to whatever feed, often oats. Gah. No. Others define it as simply a foal feed fed as a meal in a “creep feeder” so only the foal’s nose can get in there, not mom’s. That’s fine, but that’s just a delivery method, nothing to do with how/what to feed.
At 3 months, if she needed more weight (she doesn’t) I would not use anything but a milk-based foal pellet.
At 4 months you can start a regular growth feed, or a ration balancer. I used Progressive Foal’s First Foal Starter, and transitioned to their Grass Balancer. Any ration balancer will be fine. If she ends up needing more calories, then you need a good quality low sugar feed.
So in your case, you could start offering her a small amount of the Grass Balancer at 4 months, and work up to her recommended amount
Whatever you choose, get her used to it before you wean, so it’s one less change you make.