Up to 2GB, SD cards are the original SD technology. Above that (4GB or more usually), they're SDHC - same form factor but considerably different in how they're accessed.
So if you've got an older device you want a new SD card for, restricting yourself to 2GB is a much safer bet than going with 4GB.
(Irrelevant side note: There are non-HC 4GB cards, but these use a vendor specific extension over standard SD protocol, and in consequence, they work in even fewer devices than SDHC cards.)
You can try .. but on most phones to access the file system, like on Motorola, you will need software intended to work with the phone manufacuturers product .. In my case Motorola Phone Tools
If you tell us what brand phone you have, we can point you to the proper
product .. some are free, some not.