Wow, already on a 4 m sail.
Is she using a trainer type rig or a small wave sail type rig?
If she can handle a 4.0 m2 wave type rig, she could handle the 4.2 or 5.0 m2 Sailworks Retro Ripper 3 rigs very easily.
As far as a plug for the center fin box, yes all the Starboards that come with a center fin hole also come with a plug to fair it in when the center fin is not being used.
Actually, I've sailed many boards with a center fin box and no plug, and it does not impact the performance as much as you might think, plus you can always take a piece of duct tape and cover the center fin slot quite nicely.
Mostly, once you get planing, the center fin is nearly out of the water as the boad planes on surfaces further back under the board.
Hope this helps,