I never mix RAM sticks - not even by purchasing two kits (i.e. buying two packages of 1 X 4GB RAM to get a total of 8GB) - I always buy one kit (i.e. one package of 2 X 4GB RAM) because the memory is made from the same production run. Slight changes in the manufacturing process - especially when combined with two different manufacturers cause issues with RAM.
You might save £10 - £20 by purchasing this kit, and it might work properly, but you could also be wasting £14.00, and have to purchase the full kit.
If you get a matched pair - 99% of the time there are no issues:http://www.amazon.co.uk/Kingston-Memory-Toshiba-Satellite-C650-19T/dp/B009NCSI82/ref=sr_1_fkmr1_1?s=computers&ie=UTF8&qid=1358539231&sr=1-1-fkmr1