Go Disk Management to see how it allocate the 4GB SD card. 4GB only show 8MB could mean that there is some unallocated space in SD card. you may delete the partition accidently or something like virus attack, etc.
the best solution should be format it using Disk Management, if can not format the SD card with Disk management, try the third party software like AOMEI Partition Assistant.
As liailsa said, there may a partitioning issue with your card. Another option is to be facing a formatting problem. Anyways, it is a good idea to open Disk Management and check if the SD card is correctly recognized. Is the file format correct or it appears as RAW? Do you have important data stored on the card? If not, try to format. If you are using an adapter to connect the SD card, try it on another PC or change the adapter. Please test if the card is working normally when inserted in a phone, camera or tablet.