I would hold off on the thumb gaurd for a while until she starts school.
I, too, believe she will stop when she starts school.
I sucked my thumb until I was 5 also. When I got to kindergarten, there were 2 of us that were still sucking thumbs. The other kids teased us, and the teacher made us take our rest mats to the other side of the room and we could not rest with the other children as long as we insisted on sucking our thumbs.
The two of us decided to help each other. We made a pact to remind each other every time we saw the other with her thumb in her mouth. As a result, we broke ourselves of the habit in a very few days, and became very good friends in the process.
Sooner or later, your daughter will come across a motivation strong enough to make her stop. School will likely provide that.
This also reminds me of my son when he was around 2 and still using a pacifier. (He called his pacifier his "plug".) We took him to a rodeo that was in town. At the time, he thought he wanted to be a cowboy when he grew up. As we walked him around the grounds before the show started, he was in awe of the horses and bulls and the cowboys walking around. Hand-in-hand with his Dad, he grew pensive. Finally he looked up at his Dad and asked "Dad, do cowboys suck "plugs" ?" His Dad calmly and in in a thoughtful tone (holding back the urge to laugh, of course !) said, "No. No, son, they don't." Nothing more was said. My son thought about it a second or two longer, and with a serious face he pulled the "plug" from his mouth, threw it in the dirt, and walked on, never looking back ! LOL !!
I was so proud of my little man, but so worried that when bedtime came he was going to regret his decision, and we would have a rough night. Not so ! He never asked for it again !
I was a big believer in not forcing children through these milestones before that day, but that belief is written in stone in my heart and mind now. Each of my children found their own motivations to move past bottles, pacifiers, and other childhood habits all on their own, and none carried these habits beyond a reasonable age. I think stressing a kid over a security habit can have the opposite effect of making him need the security it provides all the more.
It all boils down to motivation, motivation, motivation !!
:o)