laptop memory

I have recently got an Asus A53E-ES31 (K53E) from the US.
It has one module of Elpida 4GB 2Rx8 PC3-10600S-9-10-F2 RAM in one slot, and could take another 4 GB in the other.
I know that it should be fitted in matching pairs, but can I maybe just add 4GB of Crucial RAM to the other slot?
Crucial scan offers a couple of 10600 - CL9 modules, one 1.5v and the other 1.35v.

I also plan to replace the 500GB HDD with a 128GB Crucial M4 SSD.
Many thanks,
Michael