Use any partition manager to destroy all the partitions and then format with FAT32. BTW, you need to remember that SD Cards use the K, M, and G conventions, not the IEEE Ki, Mi and Gi. So a 4GB card will be 4,000,000,000 bytes, not 4,294,967,296 bytes (4GiB). If the partition manager doesn't show memory space remotely close to what is written on the card, then you are stuck with a cheap Chinese rip-off. I have seen a lot these in the market in recent years.
Good luck!