Firs, it is recommended that a dual channel kit (2 x 4 GB) be used to ensure module compatibility. Otherwise trying to match individual 4 GB modules may take a while in order to find two modules that are compatible with each other.
The Crucial memory quoted (CT51264BF186DJ and CT51264BF160BJ) are NOT compatible with the Elitebook 8540p according to Crucial memory search.
Compatible memory are shown below -
Crucial
Corsair
PNY