"I'd like to know if the 4GB HC card will work in my phone"
Put it in the phone and see if it works then.
If it does then create a folder on your computers desktop (right click on a blank part of the desktop and right click, select new >Folder). once you've done that insert the 2Gb card into your card reader and then click start > computer and double click on the icon for the memory card. once the card window has opened click in it once and press CTRL + A to highlight everything on the card, then press CTRL + C to copy the contents of the card. now close all open windows and double click on the new folder you created earlier and then press CTRL + V. After a couple of minutes the contents of the 2GB card will be copied to the new folder on your desktop.
Once the files have copied over you can remove the 2GB card and insert the 4GB, now you simply reverse the above to copy the contents of the new folder back to the 4GB card. (so open the new folder, press CTRL + A followed by CTRL + C, then open the 4GB card and CTRL + V to copy everything back to it)