Q.Meyer lemon tree
We bought a Meyer lemon tree about 2-3 months ago and it was about 4′ tall. The fertilizer recommended states to fertilize May/June (on established trees). This tree came with a root ball and has lemons on it. Is this an established tree?

Is this a container tree or one you planted in the ground? Given that you mentioned the root ball, I am assuming the latter. A tree may take 1-2 years to become established once it's newly planted in the ground, provided it is given proper care. During the period of becoming established, trees take some time to start growing well in their new home.
For more information on growing meyer lemon trees, please visit the following link:
https://www.gardeningknowhow.com/edible/fruits/lemons/growing-meyer-lemons.htm