Usually, laptop resellers will install 4GB ram on 1215B but in actual fact, 1215B is able to accommodate a maximum of up to 8GB of ram... YES! You see it right... 8GB!
But the motherboard only accept certain type of 8GB ram, particularly rams which are PC3-10600 compliant memory.
An example would be the Corsair 8 GB DDR3 Laptop Memory Kit CMSO8GX3M2A1333C9
Amazon.com is currently selling it at around USD$50.
Here is the link to one of it: