Introducing sippy cups

My daughter will be 5 months old 10-31, and has already started solid foods per her ped's request due to weight gain issues caused by acid reflux.

To the point; I had a Playtex sippy cup stored at my mom's house from things that we received at the baby shower. The packaging said 4m+ so I thought I'd give it a whirl! I put her formula in it, handed her the cup, and she took to it instantly! She's finished a six ounce bottle completely alone, and seems to love it!

My question is, is she too young to start on a sippy? Should I wait the extra month? I assume it's fine since she shows no issue with it, but I want to be certain before I continue usage.