AMD athlon ii x2 220 to x4 640

Well it would be a good thing if you did upgrade the BIOS to the latest,

If you are getting it by itself without the heatsink, you would need thermal paste and cleaning alcohol to clean off the old paste and install the new paste so that the new CPU doesn't have cooling issues.

If you are getting it boxed with a cooler/heatsink then you don't need to do anything other than manual labor (changing CPU and heatsinks).

If it's a socket AM3 mobo you should be good to go. If it's an AM2+ check for the latest bios update.

What board do you have? You may or may not need a BIOS update: check the CPU support manufacturer's site and compare the supported BIOS versions for the x4. If your BIOS version is under that you do need to update, otherwise just swap it out.