Should I buy all new memory or combine with old?
The 8GB would be via two 4GB modules.

As far as replacing vs adding, the DIMM slots are dual channel so you'll just want to make sure they match. Look again in System Information at the speed column. It will likely say "1066 MHz". In that case, when shopping around, just make sure you're looking for a 1066MHz DDR3 4GB 204-Pin module. It doesn't have to be the same brand.