My brand new SanDisk memory card is losing memory

I recently purchased a Nikon D5100 and have been taking pictures on the 4GB SanDisk provided. However, I have noticed that the memory available has gone from 4GB down to 1.09 GB in just 1.5 weeks. I have been plugging the memory card into my MacBook Pro and copying the images/videos to a folder, then deleting them from the card. Is there a way I can fix this or reset the original format of the card?

You just need to reformat the card. (This will dump its current contents.)

These type of memory chips have a limited number of write cycles. You need to leave the pictures on until the chip is about full, then wipe it clean and repeat. Then when it starts having issues, throw the chip away (or just retire it).

I recommend you invest in a USB cable to go between the camera and the MacBook. If you plug the chip into the MacBook, it is apparently altering the file system, and adding contents. If you run off the cable, the MacBook will just ask the camera for contents. It will be a little slower, but the chip will last much longer.

Have you ever bothered to format the card? In the camera NOT the computer? Like your manual tells you to do? Do not delete them, FORMAT the card instead. Copy your images to your computer then format the card in the camera every time and you will think you have a new card.