4 months u say..? mine ive had Almost 2 weeks and hes about that size if not bigger... i dont think mine is much older then 3-4 weeks maybe 5. i could be completely wrong but..
When i got my beardie she was about 3 months old and about 10 inches or so. she's now 6 months and when i last measured her (about 2 / 3 weeks ago) she was roughly 12 inches or so.