It only takes 2gb - http://www.cnet.com/products/acer-aspire-one-d257-13473-10-1-atom-n570-windows-7-starter-1-gb-ram-250-gb-hdd/specs/
MEMORY
Max Supported Size 2 GB
Technology DDR3 SDRAM
Speed 1066 MHz / PC3-8500
Form Factor SO DIMM 204-pin
Slots Qty 1
Try - https://www.ramcity.com.au/upgrade/acer/aspire-one-netbook/d257-aod257-xxx/1192?adgroup=5661310442&adposition=1t1&campaign=109867562&device=c&keyword=%2Bacer%20%2Baspire%20%2Bd257%20%2Bmemory&location